It sort of reminds me of Ed Norton in Primal Fear. Evil bastard who acts like a simpleton with a split personality and gets off scot free..
I'm not all that well versed in what is/isn't legally considered insanity. But does the fact that he was quite lucid come into play?
The well orchestrated and meticulous planning, the immediate retaining of legal counsel, the incredibly calm demeanor immediately after destroying a dozen human lives.. All of that is more unnerving to me than if he had been slobbering and shitting himself while waving around a gun. All of the mentally ill people that I've known, haven't exactly been aware of it. This guy seemed to know exactly what he was doing. I think people are sometimes capable of shutting off their humanity and performing heinous acts that I can only describe as evil.