Drawing a line for mental health is hard. Where would you put it? Are religious people sane? Helicopter parents? Fanboys? People with fetishes? All that isn't exactly completely reasonable. It's been said that sanity is more about consensus, than rationality.

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.