We often hear that abortion is an evil of extraordinary moral depravity, so much so that it trumps other forms of murder. I think that’s right, relatively speaking. Let’s take a typical case of legal abortion wherein a woman chooses to have her child murdered (the euphemism, I believe, is “terminate the pregnancy”) by a licensed physician with whom she has entered into a contractual and financial agreement.
The physician who actually performs the abortion is morally guilty of murder. The mother, though she did not actually kill her child, consented to and willingly arranged the murder, which results in what the Catechism describes as her “formal cooperation” in the evil of abortion (par. 2272). However, her moral offense is exacerbated by the fact the severed the “natural bonds” between mother and child. The Catechism reads:
Infanticide, fratricide, parricide, and the murder of a spouse are especially grave crimes by reason of the natural bonds which they break. (par. 2268)
It appears that the moral culpability of the mother is greater than that of the physician who performed the abortion insofar as the mother is guilty of formal cooperation in murder and has severed natural bonds through infanticide. Unless, of course, the physician is father or sibling to the murdered child. At least, this is how I understand the Catechism on this point.
This all suggests that, from a moral standpoint, abortion (along with other acts of killing family members) is an especially evil form of murder, graver than murdering friends or strangers.