States get to define crimes. Perhaps, then, it isn't unconstitutional to define the acts of insane people as crimes -- even if what they are doing in qualitatively different from what sane people convicted of the same crime are doing. But, fundamental justice is offended when an insane man is prevented from arguing at trial for knowingly killing a police officer, that his insane delusions prevented him from knowing either that the man was a human or a police officer, and that his acts were typical of someone in his shoes.
Maybe the U.S. Supreme Court got it wrong. Maybe it is Arizona, Delaware, Georgia, Louisiana, and Ohio that are screwed up. Either way, they've got it wrong. Not allowing defendants to present evidence relevant to the state's prima facie case in a criminal action is just plain wrong.