If Hitler had been aborted. If say, Chas Manson had been executed, How many tears would you have shed?
You cannot predict, before someone is even born, what they will do with their life, or what impact they will or will not have on others.
You're talking about putting people to death for crimes that you think they might commit in the future. Under all accepted standards of criminal justice, you don't get to punish a criminal, much less put one to death, for a crime, until that criminal has, in fact committed a crime, and been proven guilty of so doing.
Beside, although this is an argument for another discussion, I very much doubt that preventing Hitler from being born would have made much difference. I see him, really, as someone who was little more than a figurehead for a movement that was going to happen with or without him. If Hitler had never been born, then the Nazis would still have risen to power in Germany, they still would have tried to conquer all of Europe, and perhaps even set their sights on conquering the rest of the world after that, and they still would have carried out the massive effort for which they are most infamous, of trying to exterminate Jews and other
untermenschen. I think history would have played out pretty much the same as it did, with only minor variations, due to different influences, from a different leader in Hitler's place.
And now, so many decades later, I think the only difference that Hitler never having been born would make is that we wouldn't have Volkswagen. The
KdF-Wagen, which gave rise to Volkswagen, was one of Hitler's pet projects.