The main way to be evil in this game is to go to a lot of people who give quests.
When they offer you the quest, say "Hell no" and shoot them in the face.
Otherwise, just run about being a dick.
If you see a bunch of raiders picking on some innocent wastelander, kill all the raiders and shoot the wastelander in the face before he gets time to thank you.
Basically, to be evil, shoot everyone in the face.
If you get chased down by Regulators (wild west sheriff types), you know you're doing a good job.
If you get chased down by Black Talon mercenaries, you need to shoot more puppies.
Oh yeah, the most evil thing I've seen someone do is:
Blow up Megaton
Go to Underworld to find out Moira is still alive, just turned into a Ghoul.
Let Moira know that you hate her and tried to kill her.
She runs, you chase. The hunt is on.
For some reason, even though she is damned crazy and annoying, she seems like the nicest person in the Wasteland.
