Killing hitler would not have stopped nazism from existing. Killing mussolini would not have stopped fascism. Someone else would've stepped up instead, it would have a different name, different facial hair, but nazism would still develop and cause a war of unprecedented scale and untold amounts of suffering. It wasn't a person that created those movements, it was resentment, a palpable threat, the anguish of a people torn in a disastrous post war enviroment. But just that wasn't enough. People didn't elect Hitler because they resented the Versailles treaty. People didn't vote Mussolini at all. They were facing the existential threat of a rising movement in Europe, that convinced even the most reasonable peasant that the march on Rome was not just acceptable but favourable to the alternative. They were faced with rampant Communism, beating people in the streets, proposing preposterous ideas that went against the very core of the people they were supposedly fighting for, vandals and thugs and terrorists. So violent in fact that the authorities nor the people intervened and even cherished once the tables turned, and the fascists went from getting beaten in the streets to organizing death squads to fight off the bands of communist terrorists. Why would the police care if the thugs throwing bombs into police stations get beaten to death? Why should I care if the socialist that set the building where my dad works on fire because they let a politician talk there?
The fascists didn't get in power through legitimate means, but the communist threat was so great that nor the authorities nor the people objected. It wasn't because mussolini or hitler were in some way particularly gifted, except maybe somewhat good at giving speeches.
Long story short, if you want to travel back in time to stop WWII, don't shoot baby Hitler, shoot baby Marx
