To kill a child in the womb is unspeakable; a crime against God, our humanity, future generations, and the most innocent of all. See also "Sin - Abortion" section.
Certainly one can understand the Holy Spirit helping the adult John the Baptist in his special role. But is there a need for the Holy Spirit to do so while John is still in the womb? Only if the "mass of cells" within the womb are indeed human (they do have unique human DNA, of course) and in need of the Holy Spirit's continual protection.
In the ancient world, killing the unborn was punishable by death. Here, the death is accidental (albeit recklessly so), and still to be severely punished.
