Killing babies is taboo for us, but not for them
--
Modern first world societies can "afford" to pay the costs of physically/mentally disabled children and adults.  There is enough excess resources to support those who may not be able to support themselves.  Special programs for handicapped children help teach them life skills. Modern Western society also benefits secularism that looks for scientific, rather than supernatural explanations for phenomena. However, in a society where resources are very tight, or secularized science doesn't exist, the ability to "afford" the extra costs of a handicapped child may not be possible.

Many human societies throughout history have practiced systematic infanticide.  A quick reading through the [Infanticide][1] Wikipedia entries shows that it's a very old practice.

Humanizing Infanticide
---
The easiest way to humanize something is to show *why* it's done.  Many people live in circumstances that never have to ask whether a baby should live or die.  In crafting the story, if the author can adequately describe the circumstances that contribute to infanticide then that will help lead the reader to the same conclusions that the character arrives at.

The babies are being killed because of resource constraints, then describe in a vivid detail as possible those constraints.  If they are killed because of cultural traditions/misunderstandings then describe that culture.

There is a fine line to walk on this topic.  Advocating infanticide is different than describing it.  The author will need to be very careful to describe the practice of infanticide and the reasoning of those who do it, without condoning that behavior.

You want the reader to say, "Oh, I get why you would do that.  It's horrific in the extreme but I get why you do it."


  [1]: https://en.wikipedia.org/wiki/Infanticide