Megazone
On his last warning
I don't think it is difficult to say. He knew himself that he had evil thoughts and always intended to commit mass murder.
He was sane enough to gain a position of trust within an airline over many years. That takes some doing if you really are a loose cannon.
I would even go so far as to say that this was his ultimate intention before he even booked his first glider lesson.
What if he'd started developing temporal lobe epilepsy whilst dealing with the stress of his job?
A type of epilepsy which many get confused with schizophrenia or depression. Often they might hear voices in their head telling them to do certain things that may scare them. It often effects them in their sleep too.
Like I've said, so many possible factors.