Would you have objected in people during pre war Hitler Germany started killing Nazi officials? Would you have objected to someone killing Hitler before he came to power?
When they were teenagers? Uh... yeah.
I've read a lot about Hitler, including The Rise and Fall of the Third Reich, cover to cover, at least twice. In that book, William Shirer researches Hitler's past back to his grandparents, and even before.
Not one mention of anyone thinking he could be a future mass murderer when he was a teen, or even in his twenties.
Maybe if you had a time machine, that would give you absolute certainty who to kill.
But the fact Hitler made it to adulthood and implemented his dark vision is proof that time travel is impossible.