Gromit - //Not making excuses.
Trying to work out why he didn't jump in front of a train and instead slammed a jet into a mountain.//
That is rational thought - I will kill myself and cause minimal pain and upset to everyone around me.
But suicide is not a rational act. The man may have decided right then and there that it was time to end the suffering, and he would not have thought about the other people on the plane.
We always try and apply rational reasons to such dreadful actions, but we shouldn't because it is a futile exercise.