If Adolf Hitler had died while fighting during the WW1, who do you think would have lead Germany and the rest of the world into WW2? Do you possibly think WW2 would have not occurred at all?
Given the harshness of the conditions imposed on Germany after WWI and the revanchism they engendered, I think WWII was very likely to happen even without Hitler.
The scary part is that if you remove him from the scene, you might have ended up with a party resembling Italy fascism in Germany. Devoid of the anti-semitic madness, a fascist Germany might have kept the scientists that made the atomic bomb working for them. Once Germany had the bomb, the Soviet Union would copy it, just like it did with the American one. Britain, France and the USA would do so as well.
Imagine WWII starting between 1945 and 1950 with a nuclear armed Germany facing a nuclear armed France and/or Soviet Union. Whole cities would be vaporized. It would make the losses sustained in the European theater of the war to pale in comparison.
Maybe that’s the reason why no time travellers ever attempt to kill Hitler.