The premise is that eternal punishment is harsh for finite offenses. However, if someone abused and murdered children within a matter of 30 minutes, any reasonable person would expect that the offender be locked away for the rest of his life. No one reasonable would argue that since his actions lasted for 30 min therefore he should be punished in the equivalence of 30 min or maybe an hour. That would be a ridiculous argument. If we expect that someone should no longer have the right to freedom for heinous crimes, but locked away for the rest of life, are we expecting them to get a pass in the context of eternity.
And the matter of hell as eternal torment is still debated within Christianity. All Christians do not hold to the notion of infinite torture. But all Christians believe in the permanency of any verdict rendered in the pursuit of eternal justice, in the same way anyone reasonable would expect the ruling of a court over a someone who rapes and kills a child would be permanent and not overturned.
No. Not even Hitler or Stalin deserves that kind of punishment. And what would it solve anyway? Would that make their victims any less tortured or dead? No it wouldn't. Its pointless and stupid. At some point their punishment would exceed their crime? And that's even in the most extreme cases. Does someone really deserve it for minor punishments?
AN infinite punishment does nothing to teach the person what they did wrong, it only teaches them that they did something wrong. It only serves the purpose to to instill fear into a group of people in an attempt to get them to behave, rather than actually educating them in why a behavior is wrong.
It's rather absurd that someone could condemn another person infinitely for a finite crime with finite results, no matter how heinous the crime. Think about it. It most stable institutions, the highest crime attainable is life without parole. Ergo, this person suffers for a finite period (their life) for a finite crime (most likely first degree murder). Once they die, they are alleviated of their finite punishment. It seems to make sense.
However, in any concept of infinite punishment, there is no way to remove yourself of said punishment; precisely why it is "infinite." Ergo, if you kill someone, it is the equivalent of being given life without parole and not being allowed to die.