"Why is it reasonable to believe in prisons, but not in the hell?"

[FlatAssembler]

I would argue the purpose of prison is not to punish but to remove that person from society so they can't repeat the crime or commit future crimes of the same nature.

But prisons don't remove violent people from the society. They are a place from which those violent people return with even more psychological problems. Death penalty removes violent people from the society, prisons do almost the opposite.