I come to you with this question because, having read all your other metas, I think you'd be the right person to ask. Id love to know what you think about Regulus because I have a very hard time understanding his character. Partly because of fanon characterization of him makes him seem like some secret rebel against Voldemort and partly because I just can't really understand any of his motivations. But regardless, I think what we know about him in canon is so interesting - i just can piece it all together. I'd love to know what you think!
(Sorry for the longish ask)
thank you very much for the ask, @hauntingpercival! regulus is a character i also find a bit of a mystery, and so thinking through this answer was really fun.
i'll start by being clear that i'm certainly not a regulus fan. by which i not only mean that i don't vibe with the fanon!regulus of the marauders fandom, who is essentially an original character - and you can read my views on jegulus here... [spoiler alert: i do not back it] - but that when he appears in my own writing in ways i'd like to hope feel influenced by his canon form, i always find myself focusing on aspects of his character which are rather unlikeable.
there is a little bit of a discourse-y reason for this, which will be pertinent to the rest of this answer...
i really don't like the sort of "omg aristocracy is so hot and sexy and interesting" tropes which are so prevalent in writing around the black family. this is firstly because i don't think that aristocracy is in any way these things - and i find it distasteful to imply otherwise - which is because i'm a prole who lives somewhere still bearing the scars of british colonisation who also went to the sort of university where one sometimes encountered aristocrats and they were all cringe and unbearable.
but it's also because it's not - and i will genuinely die on this hill - an accurate reflection of how the blacks are presented in canon. not only does it take sirius' comment that his parents considered themselves "practically royal" to be a statement of fact [sirius is quite clearly taking the piss out of his parents' pretensions], but it also misses that the purpose sirius' discussion of orion and walburga's politics serves in the narrative of order of the phoenix is to show how mainstream their blood-supremacist views were.
sirius tells us that his parents were not death eaters, but that they nonetheless thought voldemort's overtly sectarian political aims were correct. in this, they hold the political views order of the phoenix emphasises belong to cornelius fudge - unimaginative, deferential to the class system, casually prejudiced, and so on. orion and walburga function as a way of showing us just how entrenched the death eaters' manifesto is, how close voldemort came to winning the first war, and what an uphill struggle the order faces to unravel the roots blood-supremacy has in the wizarding world.
[and they also show that the baffling vibes of grimmauld place - while these are made worse by it being three different gothic literature tropes in a trenchcoat - are wizarding norms, rather than evidence that the blacks were uniquely immersed in dark magic. the decor at grimmauld place - and the family's collection of dark artefacts - is the same as that found in malfoy manor, even at a time when lucius malfoy is considered eminently socially respectable. this is a point we will come back to...]
i think, then, that it's crucial to approach regulus not as a swaggering aristocrat, but as someone from an upper-class background which - while still posh, rich, inferring enormous social capital, well-connected - was unremarkable within the circles in which he moved.
by which i mean that hogwarts is based on real-world institutions - britain's elite boarding schools - which are so exclusive and expensive to attend that the student body are from a class-background which seems inhumanly exclusive, affluent, and powerful from an outsider perspective [i.e. from the perspective of someone from the majority middle- and working-classes] but which seems completely normal within the student body itself.
[i.e. nobody at eton with princes william and harry will have been astonished to have been at school with a royal, because they will have been familiar with their social circles, cultural experiences, level of wealth, and expectation of knowing someone with considerable social influence from childhood.]
while hogwarts appears to be a state-funded school [although it also expects an enormous amount of financial investment on the part of parents - such as buying all the textbooks], the fact that its real-world parallels are so elite [and, therefore, come with a specific "look" in the british cultural imagination] means that the student body is incredibly well-heeled and working-class students stand out enormously in a way very rich students do not. hogwarts also exists - like real-world elite schools and universities - as a way of propping up the status quo of the class system by which the wizarding world functions. its pupils have an expectation of procuring jobs in the civil service and other influential professions - using not only connections established at school but connections they possess through their [male] relatives. many hogwarts students we meet in canon are related to someone who occupies an elite position in the wizarding executive or is otherwise socio-politically influential.
at school, then, regulus would have been completely, perfectly average in terms of social position. i also like the idea of him as perfectly average in terms of intellect - and as a good, but not exceptional, seeker. this provides a really interesting point of contrast with sirius, who - while he's also not socially unusual in terms of class [and i will never vibe with tropes like him being followed by whispers going "omg, he's a black, that means he's important"] - stands out in that he's the first black in generations not to be in slytherin, that he's precociously intelligent, and that he - and the rest of the marauders - are class clowns and show-offs.
and i like the idea that this would give regulus a desire to stand out - to be considered the most important person in the whole school. we can get a hint of this in canon - the picture of sirius and his friends harry sees in deathly hallows is immediately contrasted with a picture of regulus sitting in the seeker's position in the team photo. the seeker who acts alone.
and i think this desire for notoriety is what drives him to sign up to become a death eater - that he decides he's sick of having parents with the perfectly normal level of social influence and a brother who is more popular than him, and that he thinks that he's cleverer and more worthy of attention than everyone else in the castle and the world better start showing it.
[and i've never bought - i'm afraid - the idea that he and sirius are close. it's clear from canon that regulus had no issue being thought of as "a much better son" than sirius, and that he colluded with his parents against him. sirius can love him - and miss him, and regret how they were never able to repair their relationship - but i don't think this means that he feels he's lost a bestie.]
that he holds sincere blood-supremacist views is a given - because within the world in which he lives, these are completely normal and held completely casually [i.e. that slughorn is shocked lily could be muggleborn because she's clever]. the more virulent expression of these views - saying "mudblood", etc. - is clearly considered ill-mannered, but not something which might have any real impact on one's social standing [draco malfoy uses the term with impunity while at school, and nobody ever considers that informing a teacher of this would result in him being punished; equally, nobody from the crowd who witness the event reports snape for calling lily a mudblood].
and so i think it's clear that he becomes interested in joining the death eaters - and starts putting together his terrorism pinterest board - because his mainstream belief that being pureblood is better crashes into his desire to be special to form a conviction that riding the coattails of voldemort's ostentatious malevolence is the way he can become famous.
[in this, he is very like snape.]
my assumption is that regulus is one academic year below sirius, meaning that he was born in 1960-1961. my assumption is also that he receives his dark mark while still at school - probably at some point in his newt years [so the academic years 1977-1978 and 1978-1979].
the standard view - expressed vehemently by various order members in half-blood prince - is that voldemort has no interest in death eaters who are still at school.
the order is wrong about this, obviously - not only when it comes to their refusal to accept that harry's right about draco malfoy being marked, but also in the fact that several of the death eaters who are very young at the end of the first war, barty crouch jr. [who is still young enough to be described as a "boy" in 1982 at the earliest], chief among them, must have been taken on by voldemort prior to graduating.
but it seems fair to say that admitting teenagers into his inner circle is unusual for voldemort, especially when those teenagers don't really offer him anything useful. crouch, for example, could be put to work informing on his father's movements. regulus is - as i've said - just ordinary.
and so my view has always been that regulus is marked by voldemort as a favour to bellatrix. i think this partially because i'm bellamort trash, partially because i think it's a nice narrative parallel between regulus and draco [who are very similar] to have bellatrix be responsible for regulus' recruitment when she's canonically vociferously in favour of draco's, and partially because realising that voldemort thinks of him as just some guy who warrants [essentially] a pity dark mark would be a big blow to regulus' conviction that joining the death eaters would make him impressive.
[i also think regulus is recruited before 1978 because i think there has to be a shift in voldemort's modus operandi at about this point, in order for the fact that sirius says that his parents got cold feet about what the dark lord was prepared to do after regulus became a death eater to make sense. my view has always been that voldemort's violence prior to c.1978 overwhelmingly targets state institutions and people connected to them and/or people with known anti-voldemort political views, meaning that ordinary citizens can regard these people being killed or injured as reasonable risks of their jobs and/or behaviour. and then that after c.1978, the dark lord begins targeting civilians - including upper-class pureblood civilians - indiscriminately, which makes his casual supporters start to waver a bit.]
so, let's suppose that regulus leaves hogwarts in june 1979 and finds himself expected to participate as a full death eater, after having been let off all the dirty work by virtue of being at school...
as i've said, regulus has an enormous number of narrative parallels with draco malfoy. and i think that the best way to think about him is to write him as sharing draco's canonical attitude to voldemort's cause - that he believes whole-heartedly in the message of blood-supremacy the dark lord promotes and that he has no problem with people he considers subhuman [mudbloods and blood-traitors] or unimportant [faceless families massacred in their own homes] being subjected to violence in the name of that message, but that he lacks the character traits necessary to perform that violence himself, to see it done to people he likes, or to witness what it actually involves versus the image he has of it in his head.
and so i imagine he starts struggling pretty quickly with the fact that being a death eater isn't quite as easy as he thought it would be when he was making voldemort fancams on tiktok. and that part of the reason he's primed to turn against the dark lord is because of the tension he feels warring within him at the fact that he's still a blood-supremacist, still desperate to be important, and yet growing disenchanted.
i don't however, think this is why he does what he does... so let's get into that:
why does regulus turn against voldemort?
let's be clear about one thing - regulus turning against voldemort has nothing to do with him having some sort of damascene conversion against blood-supremacy.
[or, at least, that's what i think.]
the outline of regulus' defection that we get in canon goes as follows:
voldemort asks someone to lend him a house elf. we know that regulus volunteers kreacher, because he told kreacher so - and so i imagine voldemort mentions at a meeting that he wants to procure an elf [although, of course, he doesn't elaborate on why] and regulus immediately jumps up and says "pick me, my lord" because he sees this as an opportunity to get voldemort to finally notice him.
his assumption must be that voldemort will use kreacher for a purpose which is considered normal in wizarding society - i.e. that he will require him to do something akin to domestic service, perhaps preparing potions ingredients.
it evidently does not occur to him that voldemort would transgress this social boundary and harm kreacher. not - to be clear - because i think that regulus was some kind of abolitionist legend, but because we see several characters express the view in goblet of fire that how barty crouch sr. treats winky is his own business, and that it is impolite for respectable wizards to comment on how anyone else treats his slave. this sort of social behaviour will have a second part - that it is impolite for respectable wizards to treat anyone else's slave in a way which goes beyond what wizarding slaveowners see as normal.
or: that it's fine to be lent a slave to serve you, but very much not fine to nearly kill that slave [someone else's property!] for your own gain.
kreacher informs regulus what voldemort asked of him, which makes regulus suspicious about what the object voldemort deposited in the cave was. regulus then decides to investigate.
kreacher tells us that regulus goes away for an indeterminate period of time and then returns to grimmauld place "disturbed in his mind".
dumbledore claims in half-blood prince that voldemort appears not to wear or display the objects the horcruxes are made from after he turns them into horcruxes. i think we can agree with this or not without it affecting the story - i quite like the idea that voldemort doesn't make the locket until the later 1970s [maybe after the murder of dorcas meadowes, the only person in the first war other than james and lily to have canonically been killed by him personally], but we can also say that he might have worn or displayed it when it was already a horcrux. certainly, regulus must have seen the locket - either on voldemort or somewhere in his lair - and, after kreacher tells him what happened, he goes to see if it's still there.
when he discovers it isn't, he comes to an important conclusion. one which requires a little detour...
how does regulus know what a horcrux is?
i complained at the start of this answer about the black family being portrayed as unusually immersed in the dark arts - rather than some sort of familiarity with the dark arts being perfectly normal for people of their social class.
and i am sure that you might think I'm about to have to eat my words, since i'm not going to try and deny that regulus was able to identify a horcrux all by himself...
but, actually, i'm just chucking malevolently at the opportunity to clamber onto my soapbox and say:
horcruxes are canonically not magic which only a handful of people know about. where voldemort goes beyond the theory of horcruxes which a wizard of regulus' class-background would be familiar with is that he makes seven.
this doesn't mean - to be clear - that i think it was ever common to make a horcrux [i don't think the wizarding world is quite that lawless...], but that it was reasonable to know they exist, in the way that we might have some general understanding of something macabre - like techniques for disposing of a body - which would enable us to suspect if we saw a neighbour behaving strangely while doing one of those things...
after all, slughorn can suggest [even if he doesn't believe this is what he wants to do] that voldemort could justify his interest in horcruxes by using the excuse that he's working on a project for defence against the dark arts.
that harry, ron, and hermione don't know about them is a result of a combination of their own lack of interest in the theory of the dark arts, the information blackout instituted by dumbledore at some point after voldemort graduates [and my theory as to why dumbledore hates horcruxes even in the forties? grindelwald made one - hence why dumbledore is so hopeful at king's cross that the rumours of his repentance might have been true...], and the fact that they don't discuss their mission with anyone [tonks, kingsley, and moody, who literally have to specialise in dark objects as part of their jobs, would one hundo have known what a horcrux was].
[what they would not have known is what voldemort's horcruxes were likely to be made of and where they were likely to be. it's this - rather than the idea that horcruxes are completely unknowable magic - that is why it has to be harry in charge of hunting them down: he's the only person in the series who knows voldemort well enough to realise that, for example, he'd have hidden one in gringotts because of his jealousy at being excluded from this pillar of wizarding normality.]
so, regulus has a little rummage, works out the locket has disappeared, and has no trouble - especially because voldemort mentions in goblet of fire that he'd told his death eaters he couldn't die [which regulus might not have thought was him speaking literally] prior to 1981 - guessing what it's being used for.
and so, regulus turns against voldemort.
and i think that he does this because the horcrux makes it impossible for him to pretend any longer that voldemort's aims are - when the ministry is forced to the negotiating table by his paramilitary activities - an oligarchy in which upper-class pureblood families benefit and muggleborns and blood-traitors become second-class citizens, but which doesn't deviate too much in terms of its overwhelming norms from the way wizarding society functioned at that time. instead, he is confronted with the undeniable fact that voldemort intends to reign forever as an immortal absolute monarch, and that he has never had any intention of elevating regulus and people like him to the positions of importance he so craved.
[we see something similar happen to draco, whose increasing fear of voldemort throughout half-blood prince and deathly hallows is clearly driven by him realising that voldemort isn't joking when he says that he'll kill him and his parents unless he obeys orders, but is joking when he says he'll be considered a valuable servant should he manage to kill dumbledore...]
and so his death - and his threat to destroy the horcrux - is a repudiation of his beliefs. but, specifically, it is a repudiation of his conviction that voldemort was a primarily political figure who would act as a champion of the pureblood class-system. it's him recognising that voldemort would not stop with a takeover of the ministry - he would kill and kill forever, concerned only with how much further he could venture beyond the norms of magic.