1. What the fuck are YOU gonna do in Braunau am Inn, Austria?
That's where you'll find Baby Hitler - do you speak German? Do you know how to say "where is baby Adolf Hitler?" in German? Do you know the local customs of that place and time period? Do you have any clothing that won't look wildly out of place? The point is, murdering Hitler isn't all fun 'n games ('n infanticide) - there's a LOT of homework you gotta do first.
2. How are you gonna get alone time with a stranger's baby?
Hell, even if you DO manage to blend in mildly with late 19th century Austrians, what kind of answer are you expecting from Hitler's parents? Go up to a stranger right now and ask them if you can see their baby. Odds are they will NOT let you - a complete stranger - see their baby (let alone be in a position to MURDER IT), but they'll probably call the cops on you for being a creepy weirdo who seems way too interested in trying to be alone with a specific baby.
3. How're you gonna murder that baby, tough guy?
Oh yeah, it sounds all noble and simple to say you'll kill baby Hitler. But then - assuming you're actually able to get yourself in a position where you can actually do it - you realize you're just killing A BABY. A harmless, cute lil baby, who still has all the potential in the world to do or be anything. Are you gonna strangle a baby? Shoot a baby? Drown a baby? Are you gonna look that baby in the eyes and CHOKE THE LIFE OUT OF IT?
Ever had to finish off a mouse in a mouse trap? It's horrifying. Now multiply that feeling by, let's say, TWO.
Suddenly, you're just some guy who murdered a fucking innocent baby. Say what you will about Adolf Hitler, but he didn't go around personally drowning babies.