After he received treatment and was deemed competent to stand trial, his insanity defense failed in the face of a combination of statutes and judicial decisions that make Arizona one of the most unreceptive states in the country to a criminal defendant's evidence of mental illness.
The question for the Supreme Court in an argument on Wednesday was whether the state's approach meets the basic test of fairness, or due process of law.
The court has never ruled that the Constitution requires a state to allow an explicit insanity defense, and four states — Kansas, Utah, Idaho and Montana — have in fact abolished such a defense.
But in those states, a defendant is permitted to present evidence of diminished mental capacity to negate the prosecution's assertion that he acted with criminal intent, an element that it is the state's burden to prove beyond a reasonable doubt.
In Arizona, by contrast, such evidence is barred from the trial. A defendant who relies on an insanity defense may request a verdict of "guilty except insane," which requires the defendant to demonstrate insanity under a statutory definition that is narrower than that in all but a handful of states.
Most states use either a two-part definition of insanity known as the M'Naghten rule, which stems from an English legal decision in 1843, or a modern and broader variant of the rule. The M'Naghten rule permits a defendant to show that because of a mental illness, he either did not know "the nature and quality" of the criminal act, or did not know that "he was doing what was wrong."
When the Arizona Legislature established the "guilty except insane" verdict in 1993, it eliminated the first prong of the M'Naghten definition, meaning that a defendant could not be found insane unless he could demonstrate that at the time of the crime, he did not know his conduct was wrong.
The result was that Mr. Clark, the teenager in Flagstaff, was barred from arguing that his delusions caused him to believe that he was shooting a space alien rather than a police officer. That prohibition, his lawyer, David I. Goldberg, told the court on Wednesday, meant that Mr. Clark did not receive a fair trial.
The case, Clark v. Arizona, No. 05-5966, is an appeal from the Arizona Court of Appeals, which affirmed Mr. Clark's conviction and life sentence for the crime of intentionally killing a police officer.
Mr. Goldberg offered the justices an analogy: "A person thinks he is squeezing a lemon, but it turns out to be his sister. He knows that killing his sister is wrong, but not that squeezing a lemon is wrong."
In focusing only on knowledge of right and wrong, Mr. Goldberg said, the Arizona law was constitutionally deficient in not permitting Mr. Clark to present a full defense. "A person could know in the abstract that killing is wrong, but not know that they were killing a person," he said.
Much of the debate during the argument centered on whether Arizona's narrowed definition of insanity made a practical difference. The state's lawyer, Randall M. Howe, chief counsel in the Arizona attorney general's office, asserted that nearly all those defendants who could have claimed that they did not understand the nature of their actions, the part of the definition that Arizona has eliminated, could also demonstrate that they did not know their acts were wrong.
"I won't pretend that under some theoretical, hypothetical scenario, that might not occur," Mr. Howe said, "but it is very difficult to imagine."
In any event, he said, "the state has discretion to define insanity as it sees fit."
Several justices appeared troubled, Justice John Paul Stevens among them. Could a defendant who "thought he had a mission in life to kill Martians," he asked, be convicted of murder for killing a person whom he thought was a Martian?
Mr. Howe replied, "If he truly believed he was shooting an alien, he would certainly have a very strong case for insanity." But he said that on the evidence found by the trial judge in this case, which was heard without a jury, Mr. Clark knew he was killing a police officer and, in fact, had planned the crime in advance.
The justices also had questions about Arizona's refusal to permit evidence of diminished capacity in rebuttal to the state's evidence of criminal intent. Mr. Goldberg, Mr. Clark's lawyer, said the refusal was unconstitutional "because it allows the state's evidence to go uncontested."
The state argued at the trial that Mr. Clark intentionally lured the police by driving around a residential neighborhood late at night with loud music blaring from his car. Mr. Goldberg said that under Arizona's rule, he was not permitted to argue that the purpose of the loud music was to drown out the voices that his client was hearing as the result of his illness.
Chief Justice John G. Roberts Jr. asked Mr. Howe, the state's lawyer, why the state could bar evidence of mental illness when it could not bar other types of evidence, like a defendant's inability to understand English, that might also be relevant to the question of intent. "What is it about mental capacity that allows you to exclude that type of evidence?" the chief justice asked.
Mr. Howe replied that the impact of mental illness on a defendant's ability to formulate the intent to commit a crime was "just too complex a question to ask a jury or judge to decide."
Solicitor General Paul D. Clement also argued on Arizona's behalf. While federal law applies to both parts of the insanity definition, Mr. Clement said in his brief, "the United States has an interest in ensuring that Congress retains authority to revise the standard."