This isn't the worst compared to most of the things I've done, but it is different so I'll mention it. Purposely trying to kill people in sports games. In Madden 10 & 11 I purposely kept trying to injure quarterbacks to the point that the other team wouldn't have any more. In Madden 10 while playing with some friends I purposely tried to shatter Donovan McNabb's knees and wasn't able to, so in Madden 11 I followed him to his new team, broke the starting quarterback's shoulder to get to McNabb (using Jared Allen from the Vikings) and then broke Donovan McNabb's sternum. I jumped for joy! Then I went after Tim Teebow and that bastard from the Browns and torn their knees.
In EA MMA I killed my opponent. I tackled him to the ground and wrestled my way to a good position and hammerfisted the guy to the point where he stopped moving and was covered in blood and I didn't stop until the referee pulled me off. At the end all that was left was a pool of blood that covered half of the ring and the other guy wasn't even there. He wasn't up for announcement of the winner, he wasn't on the floor, he was just gone.