"I don't think he was a bad guy" He's responsible for like 11 million deaths dude. Plenty of abused and homeless people who don't wind up killing millions of people, so that's not an excuse for Actual Genocide.
The ethical dilemma here is can we hold someone accountable for actions they have not yet committed? The only way we would be able to even consider this is if time travel is possible, but that brings in the question of how time works, which is a whole other topic I don't feel like getting into rn. So for argument's sake, time travel is possible, and what you do in the past has a ripple of consequences in the future.
If we regard the events of Hitler's life as static, then killing him as a baby makes sense because there is no way for him not to grow up and, y'know, become Hitler. Of course this also kinda throws a wrench in the idea of free will, but that's also not important rn bc once you're there, in 1889, you're not staring at Adolf Hitler, head of Nazi Germany, you're staring at an infant. Who's... an infant. A baby, hasn't done much yet, good or bad.
Is it justifiable to kill that baby because of the man he becomes?
(I'd argue if you can go back in time to kill a baby, you can also go back in time and teach the baby to not be a xenophobic asshole, but if the goal is kill Hitler while he's vulnerable, baby's a good time as any to do it.)
Personally, on the one hand if I was going to become a monster and someone came to me from the future I'd rather try and Not Do That instead of just being shot for Future Crimes... But on the other hand, Hitler and the Nazis are why several branches of my family tree are missing and I don't care how cute a baby is, if I know that baby will grow up to do that I'll pull the fucking trigger.
Fun fact: Hitler had a host of physical ailments in the last 15 years of his life and the healthcare he received was not only terrible, but so fantastically terrible that it actually gave him an even greater cocktail of problems. Think 5 hour energy vs Nyquil but on steroids (and with steroids lmao).
I still don't feel any sympathy for him. It's the least he deserved imo. But I'm aware that I'm coming from a biased position.