I recently found myself in a discussion with one of my teachers. It was a debate over Hitler(yimach shemo). He asked me, if I could "go back in time" to find Hitler as a child, would I kill him? I replied no, and cited the midrash that states that the angels pleaded with G-D not to save Ishmael because he and his descendants would persecute the Jewish People, but G-D replied that He would not punish or kill Ishmael for a sin which was not comitted yet. But my answer troubled me. What does Halacha state on a matter like this? If one were to encounter someone of whom they were 100% sure that they would commit evil or genocide in the future, or if this hypothetical situation where one would "go back in time" to find HItler(yimach shemo) as a child, would it be permitted by Halacha to kill him, knowing the evil he would commit in the future?
You were right with your proof. That is clearly the Halacha. We don't kill but only those who deserve the death penalty for crimes they have committed, and even that is a rare case. (Makot 7a) However, there is a notion of “Ben Sorer U’moreh” – the wayward and rebellious son that the Mishnah (Sanhedrin 8, 5) teaches us will be put to death young and innocent rather grow up to be put to death as a guilty murderer. This is a principle and not practical Halacha. Unless all the conditions of this wayward son are fulfilled we don’t kill. The Talmud even says that never this law came to practice and never will. (Sanhedrin 71a) One should add that in special extreme cases the Beth Din or the king do have the legal right to decree the death penalty (or other penalties) even though according to the simple law he does not deserve when they see that the law doesn’t cover this special case.(Rambam Sanhedrin 24, 4)