I've always wondered if those who commit heinous crimes aren't actually very ill and mentally abnormal. No totally sane person would do such things. Many (though not all) have had appallingly abusive childhoods or lived in severely dysfunctional families. Some may have neurological problems of the brain and so on.

I don't think one can say a person deserves to die if this is the case.
Since it's terribly difficult to know either way, one can't take the risk of administering the death penalty simply because one feels vengeful.

As to the effect on the executioner, I can only refer again to Pierrepoint's autobiography. I've read it many times, attempting to understand the man, his motives and his mindset. He seems to have been almost 'psychopathic' himself in that he sincerely believed he had a 'vocation' to dispatch the condemned 'as humanely as possible', and while doing his job correctly, he never appeared to have considered what a hugely unacceptable thing he was doing, snuffing out more than 400 people's lives in cold blood on the end of his rope. He comes across as self-important and rather pleased with himself, which I find chilling.
To me, he was as much a murderer as his 'victims'.