Wow! Dude! Spoilers!
Nah, I'm only kidding. I blew up Megaton too
There are shit loads of evil stuff you can do really. I blackmailed Tenpenny extensively concerning a matter of prejudice with a Ghoul in Underworld then I whacked his sorry ass. There was no gain in doing so, that's simply how I roll.
What I like about Fallout 3 is you can murder folks and it doesn't mean town Guards are going to be up in your grill all the time. Mainly cause there are none. You can really use it as a means to an end rather than a last resort. If you think taking somebody out will benefit you then eradicate them presently. It usually saves you time and effort.
I think the funniest mission's you can do as an evil bastard are the slave catching missions. Find Paradise Falls, it's about halfway across the map at the top. It looks like a Mall. The dude sends you off to catch specific slaves and it's glorious just how much of an evil bastard you can be with one of them. She pays through the nose when you help her then you enslave her and tell her if she doesn't run to Paradise Falls her head will explode. Battle Royale style explosive collars you see.