I just finished up my 'evil' play through of Fallout. Naming my character Charity was, I thought, a stroke of ironic genius, considering the way I wanted to conduct her through the game's various moral decision points. In the end, though, I could never choose the rude jackass options in dialog, which meant doing the Right Thing a lot where I might have opted for a more polite-but-self-interested approach, had one been offered.
In the end, though, Fallout is about saving your Vault, first from dehydration, and then from being ravaged by the super mutant army that is sweeping the wasteland. There is only so much room to veer away. You can, interestingly enough, opt for a bad ending wherein your agree to join the Master (of the super mutant army) and give up the location of your Vault and its inhabitants to his plan to achieve the Unity--his moniker for a humanity-free world where everyone is one of his new super mutants. I did that just to put a cap on my 'evil' character's story, but then went back and finished the game up the traditional way, mostly just because it's more fun to infiltrate and nuke from within the Master's lair and the Mariposa military base where the mutants are being created.
I'm still interested in finding a game that can really pull off the evil thing, though. I'll keep looking.
(pic courtesy of http://arwenevecom.ipage.com/)