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.

If you do it yourself, you'll piss off all the guards of Junktown, and they will attack you on sight thereafter. However, you also get the reward for this quest if it's Kenji who successfully assassinates Killian. You can help Kenji without getting involved in the fight yourself, e.g. by stealing Killian's weapon and stimpaks just before Kenji initiates combat or by standing in the door so that the guards standing outside won't be able to enter.

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