No, in my opinion he was not insane. It was a case of premeditated murder, i.e. a thought out or a planned action, in this case a crime, beforehand.
He intended to cause mayhem and harm innocent people by (1) obtaining a loaded weapon and spare ammunition, (2) concealing his identity dressed as the Joker (3) entering the theatre through a back exit with a weapon, (4) killing random persons in the theatre with the weapon, and (5) attempting to escape undetected through a back exit. I may be wrong but it sounds like he had a plan to me.
Hanging his a$$ will rid society of this menace and discourage others from copying his actions. You may have another opinion. Let’s hope the jury is fair to both sides and justice prevails.
BTW, in Texas, you kill somebody, we kill you back.