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.
Possibly “Making History” by Stephen Fry - although at 380 pages it doesn’t quite match as a short story, and the protagonist doesn’t stop himself so much as do something else to reverse the effects of his actions to save Hitler.