This is not a good thought experiment. A better one would be what would each side in a conflict do AND it was the most powerful nation in the world economically, militarily, culturally, and diplomatically and was the arbiter of a global system that prioritizes economic growth and benefits more from trade with a sovereign than resource extraction via explicit, systematic repression from a puppet government. The United States simply has nothing to gain and everything to lose for applying those levels of violence that you consider evil. It is actively against their interest. Enslaving Germany and Japan after WWII would be counter to their objective as enslaved populations are not as efficient or productive. A democratic Japan is the world's 3rd largest economy and is an important strategic and trade partner. A brutally oppressed, unproductive Japan would harbor resentment and require an expensive, remote military garrison to contain.
I believe that it is inappropriate to apply a moral dimension to the behavior of nation-states. In conflicts, the logic of violence is weighed between costs and benefits. When your adversary is an enemy of God, a spawn of Satan or some other type of evil, there is no moral good, no benefit in showing them mercy, it is in fact weakness and a moral failing not to expunge such evil. When you think of yourself as the "good guys", the other side is necessarily evil. Evil must be vanquished.
That's my main argument against having a "good vs evil" worldview. It becomes easier to justify horrific actions directed at evil-doers. It's a binary, and, frankly, very naive and ignorant way of thinking about the world. Even in the canonical conflict of good vs evil in the 20th century, WWII, there were heroes on both sides, "good" and honorable men such as Erwin Rommel.
One should always be wary of any group that explicitly refers to itself as the good guys. That should not need to be stated. Usually when someone says that, they're doing some heinous shit and are saying it to convince themselves that their actions are a means to an end for the "greater good"
Let's be honest, this is tribalism, a matter of where in time and space you happened to be born. You are a westerner. Your side, where you are born, is the side that is the force for good. Try this thought experiment: imagine you were born in the ungoverned area of Pakistan, and, while attending a wedding, a faceless drone drops a bomb on the reception. Are you going to blame uncle Nazir, who came out of hiding to attend? Maybe he bears responsibility, but most bystanders will blame the perpetrator of the bombing. If one happened to be around a CIA station director when a suicide bomber detonated, I doubt many people are going to blame him or the United States for that.