Angel!
This guy right here.
I'm not even really talking about the soulless Angelus, who was one of the biggest bastards ever. Cuz when he has a soul he's a pretty standup guy, doing the right thing almost always.
...til the end of the show, that is. At that point he realized in order to defeat a powerful evil (or at least give it to 'em good) he had to do some pretty fucked up stuff himself. He spent his entire run on Buffy and Angel staring into the abyss, and finally at the end it stared right back at him.
In order to destroy the Circle of The Black Thorn and prevent (or probably slow down) an apocalypse that had been in the work for centuries, first Angel sent his crew on a suicide mission. He actually ended up murdering an ally of his (pretty nice guy too, and this dude had no idea he was about to be killed) just to fool the Black Thorn into believing that he's on their side. Signed away the possibility of being human again, and to top it all off, he allied himself with an old enemy named Lindsay, while plotting behind Lindsay's back to have him assassinated after they used him, because he'll "never be part of the solution". Angel ordered the assassination to be carried out by the way by one of his most innocent and unassuming allies Lorne, and he ended up pretty fucked up over it. Just to make it completely messed up, Lindsay (before being killed) was talking about how it's kinda cool being one of the white hats for a change.
During the course of the show he had about seven allies. Three of them ended up dead, one ended up dead with her soul completely annihilated, one was mortally wounded, one finally said fuck it and left, and one stuck with him. Although by the ending of the show, it looked like everybody left was pretty fucked (although they did have an ancient quasi-evil god on their side so who knows).
A canonical comic follows the ending of the show but nobody should give a shit about it because it's pretty horrible and ruined a pretty powerful ending to the show.