State Can Make Inmate Sane Enough to Execute

"In 1986, the United States Supreme Court held in an opinion by Justice Thurgood Marshall, that the execution of the insane was barred by the Eighth Amendment, which prohibits cruel and unusual punishment. "

What were the Supreme Court thinking here?
Surely the issue with insanity is that it diminishes reponsibility, i.e. some lunatics aren't responsible for their own actions, and therefore get treatment.

To say that not being of sound mind makes the process of execution cruel means that the sedatives that are used prior to lethal injections are potentially unconstitutional.