Hitman

Hitmen are assassins hired by Mr. Burke to kill you if you chose to save Megaton instead of blowing it up. They will have a Contract for Extermination on their person.

Conversely, if you become extremely adept at mezzing and collaring people, 4 man kill squads will come after you. They rarely pose much of a threat though, as they're armed with kitchen knives, pool cues, and at worst, switchblades.