In Ray Bradbury's novel Fahrenheit 451, Montag ends up killing his boss Beatty who knows Montag has books in his house.
It has not been long since Montag was something of a dreamer, going about his day burning houses, and happy doing it. Like Mildred, he asked no questions about what he did. He never considered that life might have a deeper meaning beyond that of his own experiences. He certainly never imagined that he could be capable of killing someone.
The decision to kill Beatty occurs in a split second, as they stand before Montag's house and Beatty starts to taunt Montag. After Montag kills the captain, he starts to run. He just barely avoids being taken down completely by a Mechanical Hound. He staggers and hops to his garden where he had hidden some books. As he is making his way down a nearby alley, he becomes aware of something that makes him crumble where he stands.
Something inside had jerked him to a halt and flopped him down. He lay where he had fallen and sobbed, his legs folded, his face pressed blindly to the gravel.
Beatty wanted to die.
On the ground crying, Montag ponders what would have made Beatty act as he had. What would make Beatty so desperate to die that he would have no desire to save himself, but instead taunt Montag until his fellow-fireman pulled the trigger.
Montag has never shown aggressive or violent tendencies. He carried out his job easily enough. We should recall that he felt comforted that he burned things and things didn't feel anything. It is for this reason that he struggled so much at 11 No. Elm: usually the firemen would arrive and the owners were gone. In this situation not only was the woman there, but also she struck the match that burned her books and house, and took her life. Montag's devastation over this event supports the idea that he is not a violent person.
There on the ground, he finally comes to his senses; Montag knows that he...
...hadn't wanted to kill anyone, not even Beatty. […] He bit his knuckles. I'm sorry, I'm sorry, oh God, sorry...
While Montag recognizes through self-examination that he is not a violent person, he also realizes that everything is different. His world no longer has room for his previously passive behavior—not if he wants to survive. Not only has Montag seen things that have changed him forever, but he has also committed the most serious of crimes. He is now completely obligated to following through on this new course. He has no other choice…he cannot go back.
Perhaps what allows Montag to keep going is not the realization that Beatty wanted to die: I believe this simply puzzles him beyond comprehension. However, now Montag realizes just how much is at stake. War has been declared, he is on the run and he understands now that if he does not stop those in his way, they will most certainly stop him. It is the law of survival of the fittest.
You must remember; burn them or they'll burn you…Right now it's as simple as that.
It can be argued that Montag's insight about himself is that he wants to survive. He also wants to be a part of a world that does not tell him what to do and how to live each day. While he understands that he is not a violent man, desperate times call for desperate measures. In order to survive and exercise a right to choose for himself, he must take part in behaviors that at one time he would never have considered.