HI SO, I KNOW THIS POST IS A LITTLE OLD BUT I WANT TO ADD MY OWN THOUGHTS BECAUSE YOU ARE BOTH ABSOLUTELY CORRECT.
The thing that sets killer apart from the rest of the bad guys is that he's actually loyal to Nightmare. I personally like making the characters try and stay as close to canon as possible, and I don't truly think either Dust or Horror would willingly work for Nightmare. Dust canonically doesn't like killing and while Horrors a little more complex when it comes to violence—I don't think he's that much of a fan about what he's doing nor' the others. (He'd probably look down on both Dust and Killer for murdering their own Papyruses.) So, I honestly think that both Dust and Horror aren't loyal to Nightmare. And are only there because they're forced to a certain degree if that makes sense. Which is why I think Nightmare prefers killer so much more. Killer *is* the perfect right-hand man. The perfect "tool" in Nightmares eyes.
I never quite managed to accurately describe how I think their relationship would be, but you saying that killer would idolize nightmare to a degree has perfectly nailed it on the head. Because what does killer have left that isn't nightmare? Nightmare *saved* him in his eyes. Even if it was just to make him work with a deal—That's better than being left alone in a dying au. Isn't it?
Another thing I want to add is *how* I think their relationship would start. Specifically with nightmare
Dude, nightmare was brutally bullied and beaten as a child. The only positive interaction he ever truly had with someone was with dream at that time. Which is obviously now in bitter memory for him considering his relationship with his brother now. Killer would be the first person to truly idolize nightmare in the way he's craved for years.
Sure, Nightmare can terrorize aus and make others quiver and kneel before him. Force the respect out of them, but they'd never truly respect him in the way he wants. It's never genuine, just something done out of fear of him. Killer? Killer would be the first to do that but not out of fear or hope to have Nightmare leave him alone—Its done because he truly is loyal to Nightmare and genuinely respects him. Which I think might make Nightmare catch "feelings" in a way. Because now he's doing a new emotion so much more addicting than negativity to him. Because it's something he didn't realize he still desperately craved.
And even though Killer is almost guaranteed to stay loyal to Nightmare—The thought of Nightmare using killers respect and idolization against him is also fitting. Threatening to find someone more useful to him if Killer ever ends up failing a mission miserably. Or even trying to "reward" him by letting killer actually feel emotions with his soul. Give him a chance to feel anything other than utter emptiness even If it's only for Nightmare to use for his own benefit.
Thank you for coming to my own tedtalk, in conclusion NIghtkiller deserves better than what they're characterized as
nightkiller had so much potential but yall had to turn killer into a slutty twink