It is possible that the copilot might have worn his headset to try to reduce ambient noise, and sit in the seat with his eyes closed blanking out external noise as much as possible and staying oblivious to sight and sounds during the descent - as part of his executed plan.
True, but he would still had heard the door entry requests and the EGPWS alerts (which obviously are designed not to be masked by even ANR headsets). I guess there are no stats, but how likely is it that someone committing suicide and committing mass murder can manage to breath normally whilst slamming into a mountain at c. 400 kts?