Stop pussyfooting. You are disrespecting the innocents he murdered.
Mental illness/depression leading to suicide is common enough.
To commit mass murder at the same time is not, and indicates an extra motive.
If he rolled it on its back and pulled to vertical I might accept the 'sudden overpowering impulse' theory. But this was nothing like that. He locked the door and drove it in. The guy clearly acted with intent.
The only aim I can possibly ascribe to that is the intent to punish his employer, or the people on board.
I doubt it was the latter - given they were mostly complete strangers.