you can figure that out for your self - its just there to say you killed person x your bad - your character can always regain the positive karmatic effects of doing good deeds. its the same as - kill a person in oblivion gaurd sends you to prison
Karma and Law are two different things. Karma is like the fame and infamy stat in Oblivion, except it is completely reversible. You can blow up an entire town but get all that karma back by donating water to a bum. In Oblivion, you murder someone in front of others, one way or another you will have to pay for it.
In Fallout, if you kill someone, all you have to do is run out of town for a few days for them to forgive you.
There is no reason I've found for being a good character in Fallout other than having that warm, fuzzy feeling. In Oblivion, at least to some degree there are consequences for doing crime.