Stop Gizmo

 is a side quest in the original Fallout.

Walkthrough
Once you have successfully procured Gizmo's confession and delivered it to Killian he offers you the chance to join him in killing the murderous Gizmo. If you accept, you should go and talk to the Junktown guard Lars in the Entrance area. When you are ready you will be transported to Gizmo's office with Killian and Lars and you will be attacked by Gizmo and his bodyguard Izo.

With both dead, most of the crime in the town is gone and you will receive your reward.

Reward

 * 600 XP
 * 500 bottle caps.