In HPMoR, Moody says-- regarding casting AK-- that it's easier to do after the first time, and that might be interpreted as saying that only the first time you cast it do you have to muster up a deep, personal hatred. Afterward, a more generalized hatred seems to work, which would be the case for any of the examples above. He DOES say that you need hatred, though. Again, it seems like a parallel to the Patronus Charm, since that also seems to be easier to cast once you've done it once.

Side note: what characters have been seen to cast both Patronus and AK? Snape does it in canon I think? Does he ever cast his Patronus after he kills Dumbledore?

I realize that doesn't particularly help my argument that AK's casting requirements might prevent its use on infants and it can't be taken as any kind of explanation for how AK is shown to work in canon. But I think you do still need to want the target to be dead, and that might be a higher bar to reach with an infant.

I just wanted to point out that we don't really have a lot of data on how AK works or if it works on infants specifically. So in order to explain what we see as an anomaly (an infant surviving the unsurvivable Killing Curse), we don't necessarily need an explanation like a mother's love protecting the infant or an unknown and mysterious new Deathly Hallow. The AK having a built-in protection against its use against infanticide is no more complicated than any of those explanations. Rather than settling on any of those explanations, I wanted to encourage people to keep thinking, because none of them sound completely right!