The Bible teaches that we're all sinners and surely, God's Holy Spirit in the world restrains the world from monsters like Ariel Castro taking over. But he's an example of what the world look like if God's restraining power wouldn't be on earth as well as Hitler, Stalin, Dhamer, Gasey, etc.
People who do not understand or refuse to accept the Biblical doctrine of forgiveness and salvation have a hard time even hearing that God's saving grace can extend to people like these. But this forgiveness is something that will always be underestimated even by the most devout believers.
The Bible states that "Salvation is of the Lord" and that only by the Holy Spirit can a life be transformed and regenerated. So it takes a lot more than pity, remorse, regret or tears to be forgiven by God. And only God knows the heart of man. As humans we only think we know what's within our own hearts but the Bible states "The heart is evil and desperately wicked, who can know it". Only God can know what is in a human heart.
People often stumble at the account of the "thief on the cross" when they read that Christ forgave him but the miraculous power of the Holy Spirit was at work at that moment, and far be it from us to exactly understand how that heart was transformed. It's a miracle of God just like the miracle of birth, conversion or even the creation of the world.
