Excerpts from a law review article:
Jamie Wilson, nineteen years old and severely mentally ill, walked into a school cafeteria and started shooting. Two children died, and Jamie was charged with two counts of capital murder. Because he admitted his guilt, the only issue at his trial was the appropriate punishment. The trial judge assigned to his case, after hearing expert testimony on his mental state, found that mental illness rendered Jamie unable to conform his conduct to the requirements of law at the time of the crime--not impaired by his mental illness in his ability to control his behavior, but unable to control his behavior. The following day, the same judge sentenced Jamie to death.
The most common reaction to Jamie's story, regardless of the death penalty views of the audience, is "What?" At the very least, it is counterintuitive to kill someone for behavior he was powerless to avoid. Whether a practice is unconstitutional, of course, is hardly determined by whether it is sensible, and the South Carolina Supreme Court has held that, sensible or not, it is constitutional. In so holding, the fact that no other defendant--in South Carolina or any other state--has ever been sentenced to death after the factfinder determined that he lacked volitional control did not sway the court.