Terrorism itself surely can be a sane act. Sanity has nothing to do with morality. Spreading terror to win your battles has been practised since antiquity, war paint, bag pipes, guerilla warfare. I would find it hard to draw the line right here. Then it wouldn't take much to say that by definition, every killer is insane.
Webster defines it as, "a severely disordered state of the mind usually occurring as a specific disorder (as paranoid schizophrenia)".
This means that they concluded that he committed the crime in a non disordered state, and his mental health did not point to any particular disorder. Just because someone holds extremely radical views (you mentioned religion, politics can also be included) doesn't mean someone is insane.
However, I'm sure it was a very close call.
No normal sane person would do such a thing, only someone who is either an extremist or an insane person would.