Why would Beatty make Montag burn his own house?

Beatty wants to punish Montag for the transgression of owning books. As he says to Montag when they arrive at his house and the firemen start breaking windows as a prelude to burning it,

"For everyone nowadays knows, absolutely is certain, that nothing will ever happen to me. Others die, I go on. There are no consequences and no responsibilities. Except that there are. But let's not talk about them, eh? By the time the consequences catch up with you, it's too late, isn't it, Montag?"

Beatty feels he has given Montag chances to reform and that Montag has not taken them. Beatty is a smart, angry man who has a sadistic streak, and he wants to rub in Montag's face the fact that a fireman is not allowed to own books. He therefore mocks and jeers at Montag, trying to show him the futility and powerlessness of his frail paper volumes against the destructive force of the fireman's flames.
Beatty's sadism—his desire to punish Montag and make him suffer acutely—comes out when he says to Montag,

"I want you to do this job all by your lonesome, Montag. Not with kerosene and a match, but piecework, with a flamethrower. Your house, your clean-up."

Montag ultimately decides that Beatty provoked him by arming him with a flamethrower and having him burn his house because Beatty had a death wish and hoped for Montag to kill him:

Beatty had wanted to die. He had just stood there, not really trying to save himself, just stood there, joking, needling, thought Montag . . .

Toward the beginning of the novel, Captain Beatty informs Montag that firemen are allowed to possess a book for twenty-four hours before they burn it on their own. However, if the fireman does not burn the book after twenty-four hours, the firemen are called to burn it for him. The policy of having the curious fireman burn the book himself indicates that the fireman has eradicated his intellectual pursuits and fully supports the government institution. Likewise, Captain Beatty wants Montag to burn his own house as a gesture of Montag's compliance and acceptance of the government's stance on censoring books. Captain Beatty might also want Montag to destroy his own house because of the psychological impact it would have on Montag. By burning his own home, Montag is acknowledging that he alone was the cause of the destruction, initially by reading novels and then by actually torching his home.