The truly moral thing to do would be to go back in time and become a significant and positive influence in baby Hitler's life, being with him as he grew and making sure that he became well grounded and compassionate and other things. And still being prepared to kill him if he turned into an asshole.
And epidemic has a good point. There is no guarantee that history would take a turn for the better just because one bad influence was removed. Had Hitler never been born, we might be having this same discussion about some different foreigner, named Chuck, who caused 6 million deaths because he hated seafood and attacked Maine. And eliminating Hitler from the equation would still leave his junior assistant assholes alive and with power. Goebbels, etc. were certainly his icky equals.