Ordinary citizens sometimes kill murderers and do so permissibly. If a school shooting is taking place and an ordinary citizen with a gun finds that the only reasonable way to stop the senseless killings is to shoot the mass murderer, the citizen is within his legal and moral rights to do so. In fact, as I have argued more than once in this blog, justice (and love) in some cases seem to demand it.
Why then shouldn't ordinary citizens take matters into their own hands and kill abortionists? One reason, of course, is that to do such is illegal. One will go to jail. But are there moral reasons beyond the pragmatic reason of not going to jail?
What if a citizen finds himself in an abortion clinic where the abortionist is about to "crush" and "crunch" a baby, in part to pay for some shiny new rims for her Lamborghini? Is the vigilante not justified, whereas the person who stops the school shooter is? Why is the vigilante not justified?
I think he is not. Here is a sketch of an argument in support of that thought. One reason for thinking not has to do with the fact that abortions are a systemic practice and abortions are legal (at least some) given legislation passed by a legitimate governing authority. One has a moral duty to abide by criminal laws promulgated by a morally legitimate political authority, where abiding by such laws does not commit one to doing something that is unjust (on the very reasonable assumption that legitimate governments sometimes pass unjust laws). In killing an abortionist, one only prolongs the time it will take for abortions to occur--killing an abortionist in a society like ours will result in a dead abortionist as well as dead babies. There are plenty of other abortionists who will step in to take her place. In addition, killing an abortionist is not like killing a school shooter in the act of shooting. Abortions are planned out in advance and do not take place randomly in the eyes of the community. There are other means one could take to stop the abortionist. In fact, there are much better means which do not involve killing another human.
Killing another human being should always be the last resort when it comes to stopping other killings. Assassinating an abortionist sends a signal to the community that the government is not the final authority (this side of Luna) in matters of life and death. To do such promotes anarchy against one's government; and the best arguments against anarchy focus on showing that anarchies cannot protect the rights of people more generally better than the government. The vigilante lacks the moral authority to take lethal action, but acts as if he is a morally legitimate authority and that the government is not. The same cannot be said for the person who stops a random mass shooting--for one, he has the legal authority to do so from a morally legitimate government.
But what if the U.S. government is not a (morally) legitimate authority, as some anarchists, libertarians, and Marxists have argued? What if the U.S. government has de facto but not de jure authority? Then the above argument would need modification.