I'm really only evil to evil factions. I'll be honest though, just a quick peek through this thread is giving me ideas. Probably won't do it though, I just lack the motivation... or sheer boredom.
Let's see, in Fallout 3... Wounding Raiders till they have one little blip of HP, stalking them as they attempt to run and blow their head off and if that doesn't happen, mutilate their body. First by decapitating them and, if a body of radioactive water is nearby, toss their head into it or just letting a wasteland critter kill them.
Oh and if they're in a building, I usually put their corpses in funny poses (I actually got one to die sitting on a toilet with his head gone) or toss their body into a bathtub. Also, since I found that one bridge hideout they hang out at, I get to slaughter them till I finally get sick of it. Also, Occasional massacres in Megaton I undo by reloading my save.
Gears of War 2, as the match ends, I especially make sure to down a Locust enemy just to execute them just before the leader board displays and in 3, if I see a bot using a Myrrah skin, I do my best to after and pull off the Lancer or flamethrower execution. I especially love the flamethrower execution.
Call of Duty, just executing enemies in second chance. I took it up to 11 in World at War because of fucking veteran.
and Left 4 Dead. I actually taunt the Director in the Dead Center finale by almost filling up the car completely and just sit there slaughtering any zombies the game can bother sending. Also, for something actually evil, if the bots are especially fucking stupid in a run and get me killed, as soon as we respawn, I shoot all of them in the back with a shotgun and let them bleed to death.
Star Fox 64, if Star Wolf happens to piss me off in a run as soon as I shoot one of them down, I keep firing on their air craft or just fire a bomb at it.
Just off the top of my head.