Ok lets use Hitler as the test case and he gets killed before WW1.
However WW1 goes ahead as he wasn't the instigator and the result is the same as Hitler didn't do anything outstanding that would have changed things.
The Weimar republic comes into existence and the world carries on, Germany secretly builds its armoured divisions, submarines and air force at Russian bases as this all happened before Hitler came to power and the world has its great depression as Hitler never caused that.
Now you reach the early 1930s with exactly the same conditions as you would have had with Hitler alive, WW2 is becoming inevitable due to the short comings of the Treaty of Versailles but instead of having a raving lunatic running what was probably the worlds best trained and equipped army you have professional soldiers.
WW2 without the randomness of Hitler's decisions, chances are you have German Jews serving in the German army so it is a million or so stronger, you have the best scientists working on German armaments, instead of slave labour building shoddy weapons you have professional craftsman and the likes of Einstein helping Germany build its own bomb to me that is a far more scary scenario than a lunatic that destroyed himself and his own country.
So I stick by the argument while there is no way of tell what the outcome would have been with different variables we know the outcome of history and while it is not perfect it is better than it could have been.