I keep thinking about why was it so weird to people that Cami had a dark backstory introduced in season 3. I do agree that it should have been introduced in season 1, like mid season or something, but to me it made a lot of sense.
I mean, her very first scene in TVD has her practically narrating Klaus' state of mind. And sure, she is a psychologist, but like...I've been to a lot of those and they aren't usually that good. Sure, she was looking at a painting, but the knowledge was immediate and it got me thinking that she just probably expects people to be able to do awful things, despite also thinking they want to be good, because that's who she is. She wants to be good, but she has a dark mind.
Cami was terrified about ending up killing a lot of people, like Sean did, because she view that as a possibility. Normal people, no matter their degree, don't usually think that about themselves even if a relative did something like this. Sure, you can think about killing someone when you're stress or angry or joke about it, but to actually think yourself capable of that and fear it? No, that's not very usual.
And her attraction to Klaus, despite knowing everything about him? Her sympathy towards him?
Yeah, the clues were there all along.