Two main reasons. First, no matter how much you punish them, it will not reverse what they have done. Second, an eternity in Hell is the ultimate punishment. There is literally nothing worse. No one deserves.
Based on that, you are essentially challenging God and religions that have a heaven and hell. Surely there are some worthy of Hell, even if your views on it are that most offenders and enemies don't, surely you would agree that Stalin and Hitler deserve to go there?