You're asking me to prove a hypothetical, which is impossible. So to be fair let's turn it around, you prove that it would have been less tragic if WWII didn't happen. The Nazis rise to power and the world does nothing as they occupy one country after another. All of Europe is controlled by nuclear armed fascists, there is no UN and the other world superpowers are isolationists.
And you conveniently avoided the issue of the US Civil War, so I'll ask again: Was it necessary and (ultimately) good? It was a tragedy for sure, all war is. But it would be far more tragic if slavery still existed in the US. So it's not that I want to see lots of people die in war, it's that there are times where it's necessary because the alternatives (including "do nothing") are that much worse.
The US/N.Korea example is interesting. We've seen a level of international cooperation and diplomacy that did not exist before WWII (It really did not, nation states were much more insular). We are witnessing, in real time, the fruits of WWII, where world powers are diligently working through all possible options to avoid conflict and to avoid a world war (e.g. the US and China getting pulled into open conflict over N. Korea).
So it's not that I think people dying in war is good, as if the more that die the more we learn. What I'm saying is that it's necessary at times, because try as we may, there was no convincing the Nazis (or the Confederates) to peacefully change their ways. Of course, every possible diplomatic channel should be fully exhausted to avoid war, and we should do everything possible to minimize death of innocents. But in cases where we've reached the end of diplomacy, war (and resulting casualties) are necessary for progress.