Kill Killian

Kill Killian is a side quest in Fallout.

Detailed walkthrough
While doing the Get Gizmo's confession quest, the Vault Dweller has the option to record Gizmo hiring them to kill Killian Darkwater for him. Gizmo will ask the Vault Dweller to return with the dog tags Killian wears around his neck as proof of his death. If the Vault Dweller kills their mark in an overt way, it'll anger all the guards in Junktown, who will attack on sight thereafter.

The Vault Dweller will also get the reward for this quest if Kenji successfully assassinates Killian. However, this is not a guaranteed method.

Regardless of which approach is used, retrieving the dog tags from Killian's corpse and bringing them to Gizmo completes the quest.

Rewards

 * 600 XP
 * 500 bottle caps
 * -5 Reputation