Why did Montag kill Captain Beatty?
Captain Beatty is really loosing it. It is as if he wants Montag to do something, to kill him. Beatty lectures Montag on the dangers of books. He chides and threatens Montag who has a flame thrower. Beatty asks why Montag felt the need to keep books. When Montag doesn't answer, Beatty hits him, knocking Faber's secret radio from his ear. Beatty picks it up, saying he will have to trace it and, "drop in on your friend". In silence, Montag switches the safety catch on the flamethrower. At first, Beatty is taken aback, but he quickly recovers and continues his berating speech, demanding that Montag turn the weapon over. Montag refuses, flips the switch, and burns Beatty to death.
Montag was going to be arrested and also Montag would be angry since Beatty forced him to burn down his own house.