Lawbringer

Lawbringer is a Fallout 3 perk that may be obtained at level 14.

Description
Once you have the Lawbringer perk, any evil character you kill will have a Finger on their corpse. This Finger can then be sold to a certain person (whose identity is disclosed when you take the perk) for caps and positive Karma.

The evil alternative to this is the Contract Killer perk.

Bugs

 * It is possible that Sonora Cruz vanishes from the Regulator HQ and does not return. Fast traveling to the HQ often spawns various combination of hostile creatures and/or robots. They are normally busy killing the brahmin outside the HQ, however they engage you upon your arrival. Due to the nature of the AI and the non-player character characteristics of Sonora Cruz, this causes her to flee from the area and not return. She can be brought back to the HQ by using the console, but must be disabled, then re-enabled to reset her scripts, or she will act continually frightened. It should be noted that if you kill whatever is outside before you enter the regulator HQ Sonora Cruz will not flee.
 * She can sometimes be found wandering around outside the HQ building.
 * Even with the Lawbringer perk, regulators in the HQ will hold a bounty notice saying that you're a dangerous person who is to be shot on sight for a reward of 1000 caps when having an evil or very evil Karma status.