This one's easy: Take a tragic event that happened to you and channel all of your fury into the game. My previous love interest recently rejected me, even after all I did for her. I had a good cry (as any REAL man would do), then went beast some public lobbies in KC (29-8 and 30-9 were my two best games, most of which were at least 20+ kills).
Trust me, it works, and it's a good stress reliever when you use those n00bs as punching bags.
*NOTE: Your results may vary based on your anger level.