Antagonists

So you want to play an antagonist? That's cool. Just remember, everyone has a reason for what they do. No one is a jerk just for sake of being a jerk. That's crappy storytelling and kind of boring to boot.
 
Remember that this is a game, the idea is for us all to have fun. Don't be a jerk. Work WITH people, not against them, to make awesome scenes of conflict. Characters should clash, not players.