I believe in God, specifically the God who reveals himself in the Christian Bible. I believe that the most fundamental reason that shooting a baby in the head is wrong is because God created and loves that baby, so to harm it is to violate the will of the most fundamental principle in all reality, which is God himself. What he approves of is good and what he disapproves of is bad, and there is no higher authority to appeal to beyond that. He disapproves (pretty strongly, as it happens) of harming babies. Therefore, it's wrong for you, or me, or anyone at any time or place, from any culture, including cultures that may exist thousands or tens of thousands of years from now that neither of us know about, to do so.

Many people who believe shooting babies in the head is wrong would give a very different reason than I do. I would agree with them in this instance, but not in every instance. Because we would not share the same standard. Because a single case study, like the one you've proposed, is not a standard.