How does Captain Beatty behave toward Montag at the firehouse?

He is aggressive and threatens to kill Montag.

Montag hands his book over to Beatty, who throws it into the trashcan without even looking at the title and welcomes him back after his period of folly. Beatty browbeats Montag with a storm of literary quotations to confuse him and convince him that books are better burned than read. Montag is so afraid of making a mistake with Beatty that he cannot move his feet. Faber tells him not to be afraid of mistakes, as they sharpen the mind. An alarm comes through, and Beatty glances at the address and takes the wheel of the fire engine. They arrive at their destination, and Montag sees that it is his own house.

As Montag comes back, he is understandably very nervous. He brings with him the book that he illegally took from the woman's stash of books and also Faber's voice in his ear. It is necessary for Faber to reassure him and to try and calm him down as he reenters the firehouse. As Beatty sees him, note what he says to the men around him with whom he is playing cards:

"Well," he said to the men playing cards, "here comes a very strange beast which in all tongues is called a fool."

Beatty clearly makes fun of Montag and of his position, and awaits the delivery of the book with his palm facing the ceiling. After Montag has given him the book, and Beatty throws it away, Montag joins Beatty and the men for a game of cards, but Beatty uses this as an opportunity to continue to lecture Montag on the dangers of books. It is clear that although Montag has returned the book, the conflict is not over.

beatty should leave montag alone because he try to changethe ceul world that live in.