Why does he collapse in the alley?

From the text, we can infer that Montag had suddenly realized that Beatty was dead, and that he had been the cause.... guilt, remorse, or possibly relief. He none-the-less knew inside that it was death Beatty had wanted.

Montag took the four remaining books and hopped, jolted, hopped his way down the alley and suddenly fell as if his head had been cut off and only his body lay there. 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. In the middle of the crying Montag knew it for the truth. Beatty had wanted to die.