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, while good characters have the opposite.
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 one was a Childkiller and a Berserker, only the Childkiller entry will exist, but its description and image will be from Berserker.
- In addition, no characters are actually flagged as bad ones, meaning reactions are decreased for all NPCs.