I'm going to be honest, I don't think this is a matter of like... the scale of the bad things they've done, but of whether or not the character rings true. The character that feels "the most twisted" to a reader isn't going to be the one that does the worst stuff, but the one that's the most believable while doing horrible things. Like, a character who berates and manipulates a friend into self-loathing but thinks they're doing it for the friend's best is going to feel more messed up to me than the girl who can't stop giggling while she cuts out people's eyes, just because the first one feels real, and the second one feels like a cartoon.
"He brutally murdered his own family and laughed!!" is so, so easy to say, and would certainly be chilling in real life, but it's incredibly hard to sell. If it's done very well, we're horrified; but there's nothing more to it than shocking brutality, there's a real danger we'll just roll our eyes and think "oh gee wow, how edgy."
If there's any reason to adjust that kind of character, I would think it's that: not because "oh wow they're TOO dark!!!" but because you want to make sure the character rings true and isn't just over-the-top for the sake of being over-the-top. Making sure you're not using shocking behaviour to make a character WOW!!! SO TWISTED!!!! but that you're building a real person whose horrorific actions reflect something about them that feels genuine.