The sheer amount of physical preparation that Holmes underwent should give lie to the idea that he's nuts. He protected himself from harm by wearing a shitload of body armor and then booby-trapped his house to take out more.
His entire intent was to murder as many as he possibly could -- and then live through it.
If that kind of behavior doesn't discern and indicate the knowledge between right and wrong (a basic defense if one is nuts, I believe), I don't know what does.
This animal needs to be put down -- within an hour after he's found guilty, of course. None of this endless appeals bullshit that takes 20 years or more to finally slip him the needle.