Heres some of the stuff I did. Hopefully you will find them as an... inspiration...
1. Take over Goodsprings. I stripped all the bodies of their clothing and stuffed them in the back of a pickup truck. Yeah, dastardly eh?
2. Equip Hockey Mask, Wasteland Settler Outfit, and a Machete. Blood-curdling screams are allowable, for added effect.
3. Help out the Powder Gangers at NCRCF. When the NCR comes, friggin' bolt.
4. In Primm, do not rescue Deputy Beagle from captivity. Instead, pickpocket his Journal and leave him tied up and/or decapitate with your trusty machete.
5. Every now and then, when the mood strikes, murder an innocent (preferably one who is alone, along a desolate road far from civilization) and hack his/her body to pieces (literally) with your machete. Leave him/her there, in the middle of the road, as a warning message to all travelers. Show the Mojave that you OWN this place!
6. Betray, betray, betray. There is a reason traitors go to the 9th Circle! Use other wastelanders to your advantage, gain their trust, and then betray them and reap the maximum possible reward! For example, I helped the NCR literally until the very end of the game, and then I took Vegas for myself! Brutus and Cassius took notes!
7. This may be rather obvious to some, but I decided to include it: In Novac, trick Boone into killing an innocent for his quest (the name eludes me). Yay! Evil!
8. Crucify Benny! Muahaha.
9. The Cannibalism perk always helps. Try eating members of the White Glove Society! Yay, irony!
10. If you get bored, target an NPC and stalk them. Follow them until you are alone, and then move in for the kill. This can open the game up to endless possibilities. You could be an assassin and your target is an influential political figure! Or maybe you are a bounty hunter, and whatever you find on your target's body is your pay for the contract. Get creative!
Feel free to add on. Hopefully this will help anyone whos struggling to find their place among the evil. As an added note, the trick to being evil is to really get into the mindset of an evil person. My favorite archetype is the anarchic, deranged serial killer, but that may not suit others' tastes. Regardless of your goals, if you roleplay it well, you will have bucketloads of fun. Good luck!
- Ostrich