All the 9/11 hijackers were likely sane, as are suicide bombers in general.
John Hinckley clearly wasn’t sane. There can be a gray area in-between (probably Kaczinski, for example.)
But there is such a thing as mental illness representing a chemical imbalance in the brain that can result in violent actions for which a person is not actually responsble. Not to say that they shouldn’t spend their life locked up in an asylum.
To me, this guy looks like a classic late adolescent onset schizophrenic. He was dropping out of a neuroscience program, from what I’ve read, so he was clearly pretty bright.
Very often, these people look for ideological justification (Ted Kaczinsky, for example, or Adam Gadahn). There seem to be some ideologies where the borderline between madness and belief is very slender indeed, and the two that come to mind ummediately are anarchism (Black Bloc) and Islam.
Ernst Becker pointed out hat the mental hospitals in France emptied out during the Terrror, because their inhabitants had finally found an ideology as crazy as they were and one in which they were perceived as normal.