My guess is that he had a plan and stuck to it, and that is what really got him killed. Surviving is about doing what you need to do to survive, not about sticking to the plan.

It seems unfathomable to me that anyone who has ever walked in snow more than a few inches deep would just wander out in the woods in deep snow without snow shoes. I have done it but after a hundred feet or so realized how stupid it was and turned back (I think I was about 19 at the time).

Its hard to know with any certainty what really went wrong, because the only guy that knows for sure can't tell us. It does appear it was a judgement problem. That's often a problem for certain personality types who just cannot admit to themselves that the course of action they have laid out might not be ideal and perhaps a different course of action is warranted.