In Fahrenheit 451, why does Beatty treat Montag in the way shown?
Chief Beatty is the major antagonist in the book, and serves to show the rationalization of the government and its totalitarian control of its citizens. His interactions with Montag are genial but very expressive and explicit; he wants to convince Montag through persuasion that the government is correct in its actions. Beatty understands that Montag is balancing on the edge of accepting or rejecting the government stance on books, and so Beatty wants to prove through argument that his ideas and philosophies are correct.
"Well, then, what if a fireman accidentally, really not, intending anything, takes a book home with him?"
"A natural error. Curiosity alone," said Beatty. "We don't get over-anxious or mad. We let the fireman keep the book twenty-four hours. If he hasn't burned it by then, we simply come and burn it for him."
(Bradbury, Fahrenheit 451, Google Books)
While the government uses brutality and fear to control its citizens, Beatty is the reasonable voice that convinces people that they are being controlled for their own good. He is intelligent and even well-read, showing how books can be used in immoral ways even as he uses them to his advantage. Beatty's interactions with Montag show that in his own mind, he is the hero of the story; he is preventing the collapse of a working dystopia through intellectual reasoning instead of by force. By the end, he is openly contemptuous of Montag, showing that he is completely convinced of his own superiority; at that point, Beatty no longer wishes to convert Montag, but wants to destroy him for opposing his philosophies.