I do think you're right that there's an extra element beyond just suffering (otherwise you can argue that killing infants instantly is okay if they don't notice and they're not yet self-aware).

I think it's a mixture of suffering and having a neural network formed enough for ...something? I have an intuitive feeling that it's wrong to kill infants before they're self-aware even if 'done painlessly', but I don't feel that way about a blastocyst or a fetus without a sufficiently formed neural network that can suffer.

I recognize this isn't perfectly consistent though and I don't have a great answer for why.