How does telling someone in a confessional box cleanse you of your sins?

Well they say confession is good for the soul don't they. So if you go by the rules of the catholic church you can murder someones mother if you like. Then all you have to do is say sorry to a priest and you are in to gods holy kingdom,sure as eggs is eggs. Ah but if it was your mother that was murdered and you can't forgive her killer,you can't come into heaven. That's the rules I'm afraid. You couldn't make it up could you. Except someone did. Who ever wrote the bible did...