You: That is indeed an interesting argument, but I see no reason for there to be a God in order for there to be justice. Fate could force bad things to happen to those who are bad, or karma could lead evil beings to be reborn into evil lives. Either of these forces might bring justice into the world, and neither of them requires the existence of one or more gods.
Aaron: Fate, karma, reincarnation...I do not believe in such things.
You: I do not believe in God, but you still try to convince me of his existence.
Aaron: Think of it this way, even if there were fate or karma or reincarnation, and there isn't, God would still have to exist because the world could not exist without God.