Look at it this way:
Imagine yourself as the Creator. Now, you create humanity, give them free-will, and explain to them that, "You can do whatever you want. All I ask is that you not violate these ten rules. Everything else is cool, just please don't break these rules."
Now, these rules are not simply there to make you feel big, they actually serve the purpose of helping your creation live in harmony with you and each other.
Well, time passes, and what do you know, your creation breaks one of the big 10. You understand that they're human, they'll make mistakes, so you set up a system so that if one of your creation does something wrong and asks you for forgiveness, they'll receive it.
Simple. Good. Just.
Now, that's all well and good for those who at least try to follow the rules. What about those who don't?
Minor offenses are, of course, to be expected. They're human. But what of those who murder, rape, pillage, and plunder.
Again, the same system you set up for forgiveness applies to all. They can come to you, apologize, try to do better, and all is forgiven. But they never do. They continue in their ways until the day they die.
Then what do you do?