The mitzva is on Am Yisrael not on any individual. An any case, according to Rav Chaim Brisker Amalek is any antisemite so baby Hitler was not yet Amalek. Certainly not before he was even born (the context was abortion). I also don’t know if that would have changed anything. Maybe someone who lived in obscurity would have become Fuehrer.
RY, actually there were attempts on his Hitler’s life between 1921 (the Beer Hall Putsch) and 1944 (the July Plot). Their failures led him to believe that he was invincible.