I read the book he wrote a few years ago, but I haven't seen the film based on it. I'm pretty sure he had no choice. It would have been a bizarre and unfortunate way to die, had he died, but so many deaths are bizarre.

If he'd had someone with him then they may have been able to use levers to unpin his arm, but this option wasn't open to him in the circumstances. As far as timing goes, he cut things pretty fine (no pun intended) because he probably couldn't have survived a lot longer.