Kill Killian

While doing the Get Gizmo's confession quest, you have the option to record Gizmo hiring you to kill Killian Darkwater for him. Gizmo will ask you to return with the dog tags he wears around his neck has proof of his death. Obviously this cannot be done without pissing off all the guards of Junktown, they will attack you on sight thereafter.

Return to Gizmo with the dog tags in your inventory to receive 600 experience and 500 Bottle caps