You have killed a large number of people. This is generally not a good thing to get known for. People from the wrong side of the track will like you a little more, however.”— Fallout and Fallout 2 description
You get this title by killing at least 25 people, and at least twice as many Good people as Bad people.
Bad characters have a better reaction toward you in dialogue. Good characters have a worse reaction toward you in dialogue.
This reputation is broken in the original Fallout. The game checks for it, but it doesn't have an entry created under the Karma tab, so the Berserker description will display under whichever heading is first. For example, if a player had Childkiller and Berserker, only the Childkiller entry will exist, but its description and image will be from Berserker.