Murder and killing are distinct - that is if you separate the notion of self-defence or protection from murder. The question is whether pre-empting murder/killing is itself a form of murder or just killing, at least in context of the OP. This would depend on your dimension of morality and outcome (In the moment or encompassing all perceivable time).
Some people refuse to do a thing if it seems atrocious in the moment despite long term outcomes, but others evaluate the long term outcomes and equate it into their perception of atrocity of the thing. This difference in perception is often overlooked in debate resulting in a complete misunderstanding on both sides.
The former group might see it as murder, the latter might see it as killing. It is "murder" to kill an innocent baby but it is "killing" to defend your life or that of millions ultimately speaking.
Anyway, in humouring the question as it was intended - I wouldn't kill baby Hitler if this were the only option, only because I am unsure of the consequences/outcomes absent Hitler which could have been perceivably worse. Nationalism is one thing (Germany, Italy and Japan during WW2) that is a debatable evil (If taken too far), but communism seems worse which is a guaranteed atrocity over time - at least in my view/limited understanding. Perhaps if someone could convince or teach me a little more about the theorized effects absent Hitler then I might reconsider.