Could you give a reason?

The reason is to troll.
The answer sounds right to the listener's ear, so that's what should be answered.

In practice he wouldn't do anything to him, and instead would write posts about free choice and posts about slippery slope arguments.

I assume the reason is obvious. If you want to ask something, please ask it yourself and don't ask me to formulate both the question and the answer. If you're torn between different reasons, please lay them out.

The hesitation is because we're talking here about a baby who has not sinned and at the moment is not pursuing anyone. Someone who holds by free choice would think that this, too, is not part of the definition of the current situation—that in the future he will kill. The fact that we know that in the future he will sin does not permit killing him, as the Sages said about Ishmael, "where he is."
(Though there it is speaking about what his descendants would do after several generations, and on the other hand one could compare it to the stubborn and rebellious son, who is executed on account of his end. But the stubborn and rebellious son is executed, in the legal definition, for having defied his father and mother, and what the Sages said—that he is executed on account of his end—is a kind of rationale for the verse.)

Or perhaps your intention is that when it comes to a catastrophe on such a historical scale, all the rules are set aside and one must do everything to prevent it (even something that is utterly unjust, since according to the definition of the current situation you are killing an innocent baby), just as in wars they acted by killing innocent civilians too.

These are just word games. The questioner's assumption is that I have certain knowledge, meaning there is no possibility that he would choose not to do it. If there is no certainty, then obviously one may not kill him.