Catholics try to deploy natural law arguments against “artificial” contraception. Let’s consider another application of natural law reasoning.
Some animals kill their young. For instance, the mother or siblings may kill the runt. The ostensible reason is to reallocate limited resources to the remaining, healthier offspring. Better to have fewer with higher chances of survival than more with lower chances of survival.
Likewise, among social animals, alpha males or females may kill the offspring of the betas. The ostensible reason is to assert dominance as well as reserving the succession for the offspring of the alphas. Social animals often have a hierarchy which confers a survival advantage on the group, although some individuals may be sacrificed for the common good.
Assuming that natural law ethics is our guide, is it permissible or even obligatory to kill a sickly human baby?
One can’t say that’s contrary to nature, for there’s abundant precedent in nature for doing just that. And there are situations in which that would up the chances of survival for the remaining siblings. So it serves a natural purpose.