Antonin Scalia, a Reagan appointee, doesn't seem to care.
"This is an execution, not surgery," Scalia said, adding, "Where does this come from, that . . . in the execution of a person who has been convicted of killing people we must choose the least painful method possible? Is that somewhere in our Constitution?"
(The quote is from here while the better overview is here. I hesitate to summarize the second article because I think you should read it. Basically, we euthanize our animals in a safer, more humane way than we euthanize our people and the prison systems don't want to admit it.)
Thank you Ronald Reagan.