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.
https://allthatsinteresting.com/alois-hitler
Alois Hitler wanted to marry his cousin immediately, however, their close relation posed some legal difficulties and they had to request a dispensation from the local bishop. The bishop was clearly also disturbed by the very few degrees of separation between the pair and forwarded the request to the Vatican, who eventually granted it (perhaps because by this time Klara was already pregnant).