In thee part 3 "Three Burning Bright" , Why is Montag able to "truly release himself" from his former persona?

Montag actually burns his whole house down. He burns Mildred's stuff, he enjoys that, and everything his former identity had. Pretty soon Montag is himself on the run, no longer the person he used to be.

Montag also burns Beatty to ashes permanently separating him from his former life.