It's a weird question because you're asking if I would personally do the killing, and that's where it gets difficult. Would I be considered a hero for killing this baby, or am I going to end up in prison? Would I be willing to go to prison for protecting humanity, without humanity ever realizing it and instead thinking of me as a baby murderer.
If I'm being honest, I would certainly try to kill baby Hitler (not sure if I would go through with it or not) if it was a completely anonymous thing and I could be certain that doing so would save millions of lives.
To the person with the first reply, it's interesting that you look at it from a legal perspective; baby Hitler hasn't committed a crime yet, therefore he should be treated as innocent. I disagree. Was that more of a first impression response; do you really believe that even in the case of Hitler he should be treated the same as everyone else, even if at the expense of millions of peoples' lives. Your point sounded good to me when I first read your reply, but then after a few moments I was like wait, I change my mind.