So one of the things I like about Warhammer in general is that every force has it's own motivation that can often times make you "see things" from their point of view, or at the very least understand why they do what they do. It's not just "these guys are EVIL!". That said, there is some seriously nasty stuff out there assailing the Imperium and other factions.
So what do you guys think: Which faction comes off as the most "evil". For me it's the Dark Eldar, they seem to relish in pain and torture and causing terror. Unlike many of the Chaos Space Marines, they are not loyal (some Chaos Marines could be said to have honor in a weird way). The Dark Eldar are conniving, cruel, malicious and utterly ruthless. To me the Chaos factions seem to attack with purpose, even if it is to serve some higher goal, Dark Eldar seem to me that they are random in their strikes and can come from anywhere.
I've often wondered about the Necrons, but they don't seem sentient enough (from my knowledge) to be really evil, they're more just....there...doing their thing.
(PS I don't play Dark Eldar, but I do like their fluff)
Edit: I added a poll, it only allows 10 choices so I grouped the Imperial Guard/Space Marines and the Inquisitorial forces. I realize it's far more complex than this, the motivations of the Black Templars are vastly different from Space wolves, but just roll with it guys ha ha.