Marvin Wilson is dead.

Texas, ever fastidious in carrying out the death penalty, executed the 54-year-old man by lethal injection Tuesday night, even though by clinical standards, he was mentally impaired.

The execution seemed to contradict the U.S. Supreme Court's landmark 2002 decision, which prohibited the execution of the mentally impaired as cruel and unusual punishment forbidden under the Eighth Amendment.

But the now more-conservative court rejected without comment a last-ditch appeal by Wilson's lawyers. That made no sense. For 10 years, a stop sign erected by the Supreme Court had effectively ended the practice of putting people on death row when they are intellectually disabled.

The court correctly pointed out in its 2002 Atkins v. Virginia ruling that people with disabilities in reasoning, judgment, and control of their impulses cannot understand what they have done, or what is happening to them, so they should not be executed.

Unfortunately, that ruling also granted some discretion to the states to determine who should be considered mentally impaired. And Texas used that opening as a free pass to stick to its propensity to execute without distinction.

Texas came up with a definition of impairment that brings to mind Lennie Small, the mentally challenged character in John Steinbeck's 1937 novel Of Mice and Men. Texans may worship great literature, but they should have used a different book - one rooted in behavioral science.

Even by Steinbeck's standards, though, many Texans would likely agree that Wilson, with an IQ of 61 and a clear lack of reasoning ability and adaptive skills, shouldn't have been put to death. The decision to proceed with his execution sets a bad precedent.

Wilson was convicted of the 1992 murder of a police drug informant. At issue was whether his actions might have been manipulated by an accomplice who took advantage of his diminished reasoning ability. In fact, the most important witness against Wilson was his accomplice's wife.

But it wasn't Wilson's guilt or innocence that the Supreme Court was asked to decide Tuesday. His punishment was in question, which gave the court a chance to stand by its 2002 ruling. The potential for a wrongful conviction due to a defendant's mental inability to aid in his own defense should make the death sentence void.

In fact, it's time to get rid of the death penalty altogether. There have been too many instances in which people were executed, only to have evidence turn up later that proved their innocence.

Wilson's case wasn't an example of that. In a sense, it was about something even more important - the capacity of Americans in any state, including Texas, to reach beyond the need to exact punishment for a crime and extend mercy to those who truly deserve it - people mentally incapable of making good decisions, or aiding in their defense.