For me, I gotta say Kratos from the God of War series. While he defiantly comes off as an asshole to everyone he's still likable in the way that he's a total BAMF. You gotta look at it from his point of view; he was deceived into killing the only two people in his life that he cherished and that brought him comfort, so basically, he was forced to grow in his rage and malevolence (an abnormally large amount for a Spartan) for many years. Once he finally completed his vengeance, he got a "job" that he fit perfectly and was content to try and forget the horrors of his past so he could eventually find some kind of inner piece. No such luck, though, as he was shortly betrayed by the gods and, eventually, the titans (which he formed an alliance with after the aforementioned betrayal). I'm betting something in him snapped with the first betrayal, and then completely broke after the others, as he decides to go on a god/titan killing spree which, inadvertently, causes natural disasters, death, and sheer chaos on a global scale. Judging by his motives, he's not so much a bad guy or a good guy, as he has nothing against the people who haven't wronged him but it doesn't mean he won't worry about their safety if they get in the way of him killing his enemies. The lesson here is, never, EVER betray a demigod and try to make his life a living hell; HE WILL FUCKING MURDER YOU!
Edit: Wow, long post, wasn't thinking about making it that long when I came to this page...
