His "contributing factors" showed no sign of being about to kill him before a panicking cop applied a choke, already officially prohibited by the department, that can cause continuing, worsening tracheal damage even after it's released.
The victim's past has precisely no relevance to the question of whether the cop's action at that moment was reasonable or even legal.
Of all the issues in this forum this one was easiest for me. The cop is at fault, he should be punished as much as possible. For the sole reason that the choke hold was a banned tactic by the NYPD. It was a move that shouldn't have ever been employed.
Sadly, I think that the victims chronic issues made something that should have never happened in the first place lethal. :(
EDIT: Though it does raise an interesting question. It was obvious that the man was fighting despite calling for a time-out.
However more to the point:
You can't shoot him for obvious reasons.
You can't taze him because he has heart problems.
You can't pepper spray him because he's an asthmatic.
So, barring the fact that the move the cop used was not permitted how were they supposed to restrain him without using those options, and despite his conditions he was a big person with surprising strength, and for whatever reason he did not stop fighting the cops. (I won't presume whether it was deliberate or fight-or-flight response.)