A necessary evil is an unfavorable thing (an evil) that someone believes must be done or accepted because it is necessary to achieve a better outcome—especially because possible alternative courses of action or inaction would be worse.
There is no worse outcome for a child than killing them…
…no matter how hard you try to convince yourself otherwise. If you can’t think of a good enough reason to crush a newborn’s skull, you shouldn’t be able to think of a good enough reason to crush that same child’s skull while living in the womb.