To complete this quest, the Vault Dweller first needs to get Gizmo's confession. Once delivered, Killian Darkwater will offer the Vault Dweller the option to join him in taking Gizmo. Should the Vault Dweller accept, they will be directed to Lars. Once spoken to and agreed to process, the Vault Dweller will be immediately transported, together with Killian and Lars, to Gizmo's office.
Gizmo and his bodyguard Izo will immediately attack, so the Vault Dweller has to battle along and kill Gizmo.
Sometimes after talking to Killian then when the man Kenji will show up he will not attack Killian, this prevents the quest from starting and so you cannot get the rewards. There is no known way around it. [verified]