Yeah because that would have completely nullified the sounds of the screams in the cabin and the frantic shouting and hammering of the captain on the flightdeck door. Don't make excuses for him, anyone who is able to ignore that and carry out such an act is a psychopath. He may have had depression but a clear lack of empathy and remorse makes him a psychopath and so does carrying out mass murder. Depressed people don't tend to take 150 innocent people with them.