Apparently the drugs used for lethal injections are in short supply, causing some states to consider going back to the firing squad and others to look for new alternatives, and causing some people to talk about cruel and unusual punishment.
They should go back to hanging them. Cruel and unusual punishment is meant to limit the punishment to that appropriate for the crime, so that petty thieves don't receive the sort of punishment that those guilty of first degree murder do.
Rather than trying to find the ultimate in painless death - which given the inability to ask the deceased anything is rather pointless - we should be be looking for quick and cheap. Forget the firing squad - send them to the long drop. Even if an accidental beheading occurs, the long drop is the guaranteed quickest and cheapest form of execution.