Convicted murderers have NO constitutional right to a 100% pain-free execution, but Judge Parker's hangman came pretty dang close. In a career spanning over a decade, be did not have a single botched hanging.
With all the sleezebags we have on death row awaiting execution, I'm sure we could develop someone with the skill of Judge Parker's hangman with just a little more practice. Heck, I'm willing to volunteer if needed.
Or, if anyone is really worried about the vermin experiencing pain, knock them out with the same drugs they use on surgical patients. It's a cinch that you don't feel anything when you're "under" in the OR. Then, while they're out, hit them with the heart-stopper. Then throw the body into the grizzly bear exhibit.