Belief in a fair universe often manifests in more subtle ways than thinking that horrors should be outright prohibited: Would the twentieth century have gone differently, if Klara Pölzl and Alois Hitler had made love one hour earlier, and a different sperm fertilized the egg, on the night that Adolf Hitler was conceived?
For so many lives and so much loss to turn on a single event, seems disproportionate. The Divine Plan ought to make more sense than that. You can believe in a Divine Plan without believing in God—Karl Marx surely did. You shouldn’t have millions of lives depending on a casual choice, an hour’s timing, the speed of a microscopic flagellum. It ought not to be allowed. It’s too disproportionate. Therefore, if Adolf Hitler had been able to go to high school and become an architect, there would have been someone else to take his role, and World War II would have happened the same as before.
But in the world beyond the reach of God, there isn’t any clause in the physical axioms which says “things have to make sense” or “big effects need big causes” or “history runs on reasons too important to be so fragile”. There is no God to impose that order, which is so severely violated by having the lives and deaths of millions depend on one small molecular event.
The point of the thought experiment is to lay out the God-universe and the Nature-universe side by side, so that we can recognize what kind of thinking belongs to the God-universe. Many who are atheists, still think as if certain things are not allowed. They would lay out arguments for why World War II was inevitable and would have happened in more or less the same way, even if Hitler had become an architect. But in sober historical fact, this is an unreasonable belief; I chose the example of World War II because from my reading, it seems that events were mostly driven by Hitler’s personality, often in defiance of his generals and advisors. There is no particular empirical justification that I happen to have heard of, for doubting this. The main reason to doubt would be refusal to accept that the universe could make so little sense—that horrible things could happen so lightly, for no more reason than a roll of the dice.
But why not? What prohibits it?
In the God-universe, God prohibits it. To recognize this is to recognize that we don’t live in that universe. We live in the what-if universe beyond the reach of God, driven by the mathematical laws and nothing else. Whatever physics says will happen, will happen. Absolutely anything, good or bad, will happen. And there is nothing in the laws of physics to lift this rule even for the really extreme cases, where you might expect Nature to be a little more reasonable.