I think the game rewards good guys and pushes you towards being good. Very very rarely will you have to pass a speech check to do something bad....all of the XP rewards from such choices are always for the "good" choice or push you towards the "good" outcome of the quest.
Shocking, seeing how FO2 was exactly the same.
It is very hard to be evil, unless you are a homicidal maniac, which is impossible to roleplay in a sensible way. "Well I guess one day this here courier just went postal. I blame the 9mm in my brain".
So if you are a crazed killer, problem being there are NO dialogue options that a lunatic murderer would say.
I was evil for awhile. Then I killed some ghouls and poof, good. So I said screw it.
Ironically the game some hate, FO3, gave some great options to be evil. Can you say slaver? Also had some good dialogue options if you were an ahole. But none of the games have had a good evil option.