I could never play the Fallout games as a mean person, I was always the 'Savior of the Wastes'. I doubt it'll be any different in this version.
Oddly enough my brother did go through Fallout 2 once killing EVERYTHING he could (people, mutants, livestock). It was fun for awhile, but then there was no one left to talk to.
I picked it up for the 360 today, but I won't get around to it until after I finish Tales of Vesperia and Eternal Sonata.
Tempest