Well what's the bad thing about doing a crime? The damage done. So, the punishment should be in proportion to the damage. You don't punish someone for being "bad" inside, and the point of the punishment isn't to cause some emotional effect.
No, that is exactly what punishment is for. Punishment is made to influence us humans, because we are the ones who do bad stuff. It is a fundamental principle of the universe that nothing changes unless pressure is applied, so we try to apply larger punishment than the damage done, hoping that it will prevent further wrongdoings from the same person.
