Yes, I agree with you, onagar. There will always be those who will do things regardless.
fear of punishment stops all those who might do something if there was no consequence for them.
so, while someone might kill even though they would hang for it, a hundred would not because they would hang for it. Take away the hanging and those hundred no longer have the fear.
While someone might kill even though they would spend the rest of their life in prison, a hundred would not because they don't want to spend the rest of their life in prison. Reduce prison sentence to 6 years...
Those who would never do it would never do it regardless.
Sort of related but obviously much less severe and significant but useful as an illustration - My dad tells me about corporal punishment in school. He says that there was a lad who used to get the cane. He was naughty. He was naughty regardless. He knew he would get the cane and he didn't care. So the cane was pointless for him. It didn't stop him. However, the other boys in the class, who watched him getting caned, my dad included) thought "bugger that! I don't want that!" So when you look at the broader picture - it does work.