After Beatty discovers that Montag is in communication with Professor Faber, he taunts him, insulting his reading comprehension and demanding that he turn over his flamethrower. Montag reacts in emotion, not thinking:
"Hand it over, Guy," said Beatty with a fixed smile.
And then he was a shrieking blaze, a jumping, sprawling, gibbering mannikin, no longer human or known, all writhing flame on the lawn as Montag shot one continuous pulse of liquid fire on him.
(Bradbury, Fahrenheit 451, Google Books)
Later, Montag rationalizes that Beatty, in depression about the upcoming war and in recognition that his acceptance of society was wrong, used Montag to commit suicide. Beatty's knowledge of a line from Shakespeare hints that he may have been as guilty of reading books as Montag. Although his death saved him from the bombs, Beatty's death by flamethrower is undeniably horrific and painful, and Montag finds himself forced to reconcile this killing, for self-defense, with other killings he has committed in the past by "just doing his job."
In Fahrenheit 451, Captain Beatty is killed by Montag. This happens early in Part Three, when Beatty, Montag, and some other firemen are called to do their duty by setting fire to a house. What Montag does not realize, however, is that the destination is his own home and that it was his wife, Mildred, who reported his illegal book collection to the authorities.
Beatty has known for some time that Montag is in the habit of reading books, but he did not take action. By Part Three, however, Beatty has grown weary of Montag's attitude (he says he is behaving like a "silly damn snob") and can ignore his illegal activities no longer. Moreover, Beatty also threatens to locate Faber, who is communicating with Montag via radio device. This prompts Montag to turn the flamethrower on Beatty instead of his own house:
And then he was a shrieking blaze, a jumping, sprawling, gibbering mannikin, no longer human or known, all writhing flame on the lawn.
This is a turning point in the novel, since it forces Montag to leave the city and thus become a fugitive.