Is Shooting Aliens Justifiable Homicide?

Yesterday the Supreme Court considered whether Arizona's rules governing the insanity defense satisfy the requirements of due process. Most states allow a defendant to be found "not guilty by reason of insanity" if it can be shown that he did not understand "the nature and quality" of his actions or that he did not realize his actions were wrong. Arizona recognizes only the latter standard, which permits a verdict of "guilty except insane." But it's not clear this difference has any practical impact. In the case at hand, Eric M. Clark, a teenager who shot and killed a Flagstaff police officer, later claimed he thought his victim was an extraterrestrial. If the jury bought Clark's story (and if it saw nothing wrong with killing people from other planets on sight), it presumably would have found him guilty except insane, since by his account he mistakenly thought the shooting was justified. The fact that it instead found him just regular old guilty suggests it did not believe him, even if it accepted the testimony of the defense team's psychiatrists that he was "suffering from paranoid schizophrenia" (something The New York Times presents as a fact).