Never mind how much of a douche you can be to your enemies in combat, we're talking about player characters who are just plain assholes no matter what you do I've got two to start.
Kratos (God of War): Let's face it, "asshole" doesn't even begin to sum up this guy's attitude. Because he was too proud to face his death in battle like a Spartan IS SUPPOSED TO, he cries out to Ares for hax, goes apeshit, kills his family, then decides to basically unleash apocalypse on the rest of the world to get his revenge and clear up his daddy issues.
Granted, Zeus is a total dick, even in canon Greek myth, but you don't fucking half-destroy the world because you're angry. And that's of course not even accounting for his utter lack of regard for the safety of innocents. (Needlessly letting that boat captain die, burning a caged dude to death to open a door, etc.)
Haseo (.Hack//GU): To open this up nicely, he's a teenager right about that angsty period in one's life. He decides to check out The World: R2, he's been hearing a lot about it, and since the old one's servers are long gone, we might as well check out the new blood. So he logs in for the first time and gets griefed by some PKing trolls. This single act of betrayal (Boy let's hope he never plays EVE Online…) turns him into a nihilistic player-killer hunter. Granted, he's great at what he does. After all, he ground out like 130-someodd levels solo in a party-centric MMO.
But when this one chick he likes gets data drained and sent comatose, he ALSO decides to take out a vendetta on the culprit. That's fine, understandable. Except after he too gets data drained and returned to level 1, he is a needlessly rude ass to everybody around him, including folks that either just want to play the game normally or generally The World casually. Oh, and he basically starts stalking this random chick, including dragging her into PvP arena matches despite her firm protests, just because she looks like his bae. He somehow manages to be more standoffish than early-game Cloud in FF7.
Oh, and he's voiced by Yuri Lowenthal and I'm sick of his voice.