Why does Keene take control of the bomb in the novel, Dust (Silo Saga)?

Keene intends to use the bomb to bring down Silo 1. The bomb is moved into position where Keene specifically seeks to destroy the computer infrastructure of the silo, but is shot down before he can detonate it by hand. Keene in turn uses a firearm to shoot the bomb and detonate it, thus destroying Silo 1 and ensuring all other silos have a chance to survive.