OK would we have won WWII faster if Spiderman and Superman were Allied Commandos?
Spiderman could really only take on about thirty or forty people, which is fine if you need someone to have your back in a pinch, but he's hardly a threat to the 3rd Reich. Basically, if both Spiderman and Superman both invaded the beaches of Normandy at the same time, Superman would have punched out Hitler (not kill, Superman doesn't kill) while Spiderman was still marching a mile out from the coast. So in fact, WWII would have been won pretty much from the moment Superman decided to enter the conflict.
My position on history being objectively better had we never had slavery comes from the fact that I see many more cons than pros. Besides the moral stain on the country and the racial divide, we also have a cultural divide between a region of the country that permanently identifies itself in terms of the civil war, and the rest of the country. While the Cold War presented us a platform to be a serious world leader, without the Cold War the paralysis and division in the United States has turned on itself and we see the rest of the world slowly moving on without us. And yes, I blame slavery for this.
Without slavery, though, our music would have sucked a lot more. Black influence in music is definitely a check in the positive column.
My take on atrocities in history is not a kneejerk one, however. If we had no war or atrocities from the beginning of human civilization, I don't think there would be a single recognizable aspect of human civilization now, and that is simply too far outside my scope to guess whether that would be better or worse. As a counter to the "slavery was bad" position, I would present an atrocity that was, in the long term, positive: the bombing of Hiroshima and Nagasaki. Nuclear weapons were always inevitable. One way or another, humans were going to get them, and the first time we used them in an attack on people was when only one country possessed the weapons, and they were just "wimpy" atom bombs instead of the 500kt-1mt monsters we have today. Even still, the world was so abjectly horrified by their use that no one has used them since, in spite of the fact that today pretty much everybody and their dog has at least a few. Be sure to knock on wood.
My position on the bombing of Hiroshima and Nagasaki being even remotely a positive thing even in the very long term is not one I air IRL, to be honest. It's more than a little cheeky for me to say that it's okay for
them to be bombed so we could be spared from nuclear annihilation ourselves.