So much for the constitutional prohibition against cruel and unusual punishment. In my opinion, the state should never have the right to murder its citizens (or anyone for that matter), regardless of what they may have done. I know this is controversial, but death is never a moral or just solution to a legal issue, regardless of the severity of the issue.
But since we don’t live in my idealized version of society, states that murder their citizens under the law should at the very least be required to demonstrate that they do it instantly and painlessly. If they can’t do that, no legal murder for them.