Obviously killing Willy(Busey) put him on a path to either the death penalty or life in prison+. The critically wounding of the police officer as well.
1. Could he have survived handcuffing Earl Frank? 2. The first robbery at the bank? 3. Was the die cast when he let Willy shoot-up? 4. Or even when he was "late" for the first meeting with the parole officer?
"It's the system, Lara. People will be different after the Revolution."
You seem to be asking two different questions: 1) what was the PONR in his HEAD, or as far as Max Dembo himself was concerned? (Was he doomed when he let Wily get high) And 2) what was the PONR as far as the SYSTEM was concerned? (Handcuffing Frank).
I'd say as far as the latter goes, he was definitely done when he did Frank like that. But as for the former (what was HIS OWN PONR), a case could be made that he was NEVER going to try to live straight...he didn't stick to the requirements his first day out, and he makes statements later on (granted, after he was already in trouble, but still) that lead you to believe he's just not cut out to live by society's rules.
I think, without being a psychologist, that had he been assigned a P.o. who was a better human being than Earl Frank, and who would have tried to understand him and granted him a bit of leash, he could have, however reluctantly, come to understand that the System, represented by his parole officer, did not view him as dirt under its feet, and possibly live Straight, at least for a time. But, things being as they were in the film without speculating "What if?", I'd say his POTR was Frank handcuffing him to the bed and locking him up (and leaving him to stew in jail for longer than necessary).
The beating he gave Walsh in the car might not have happened if the P.O. had gotten him out of jail in a more timely manner. Hoffman's uncontrollable fury was allowed to build up for the week he was in stir and finally exploded.
I think his point of no return was sometime back around his first arrest long before the movie started, but that you don't realize it until the final montage. It's possible that if someone more sympathetic than his current PO had gotten to him back then, he might have been able to go straight, but I think this latest run we saw of his was always doomed.