Bruce couldn't pull the trigger, despite everything the what the individual did to the people of the town, the fear spread, and the blood on their hands.  The way they manipulated, and used their own child. He couldn't kill the parent of someone he knew so closely, he couldn't call that justice or the right thing.   He put the revolver on the table only to have it pointed at his back. Your parent ready to pull the trigger. 

(This could be Hiram,Hal, Hiram, Penelope, or whoever...)