However, as I've written about before elsewhere, this kind of thinking does lead to the amusing conclusion that cutting off a baby's limb is more wrong than killing it (because in the former case there's a full-human who's directly harmed, which is not true in the latter case).

You say that like it's an unexpected conclusion. Which is more wrong: cutting off one of a dog's legs, or euthanizing it? Most people, I suspect, would say the former.

What happens is that we apply different standards to thinking, feeling life forms of limited intelligence based on whether or not the organism happens to be human.