Well, why kill him while he’s a baby? Why not wait until he’s shown signs of prejudicial hate that drives him to cause mass genocide? When he gains political power to do so, but right before he has any immediate plans? Politicians are killed quite often. Truthfully, going to jail in order to save scores of people is a small price to pay if you’re willing to go back into a time machine to commit murder anyhow. So it wouldn’t be a huge lose to do so.