The butterfly effects would add up and and any zygote formed would not be the hitler-as-we-know anymore, since it would be a different combination of sperm and eggs.
Who needs guns when you got a time machine? Don’t like your highschool bully, just bump into their parents back in time. Or you know, “bump” ( ͡° ͜ʖ ͡°) into their parents.
The real criticism of this plan is that whether you assassinate Hitler or just prevent his birth, it doesn’t solve the societal problems that led to the world wars. Hitler wouldn’t have happened, but the Germans would still feel snubbed by the outcome of the Great War