Introductory Comment: I wrote this explanation some years ago when asked why, technically speaking, it is immoral to use violence to end abortion. It involves a somewhat technical moral analysis, but the principle is summed up simply by St. Paul when he says we may never do evil to achieve good.
Fortunately, I have found that they are few and far between who fail to understand this.
Fr. Frank Pavone
A brief reflection by Fr. Frank A. Pavone
A discussion on the use of "force" and "violence" in the pro-life movement involves a discussion of the use of certain means to achieve an end, and a determination as to whether choosing those specific means constitutes a morally good act.
It is necessary, then, to comment on the elements of the morality of an action, and the relationship between ends and means.
A fundamental axiom of Catholic moral theology is that an action is not simply good unless it is good under all aspects, whereas it is evil if any one aspect is evil. In this sense, it is like a telephone number. If all the numbers are correct except one, you still get a wrong number.
The three basic aspects (or "fonts") of any action are its "object", "end", and "circumstances."
"Object" refers NOT to "motive" but rather to what the act is. This does not simply mean the physical nature of the act, but the primary moral significance of it. For example, a picture of someone taking a TV set out of an apartment does not yet tell us what the act is. It could be moving a TV which one legitimately owns to another location, or taking it to a friend as a gift, or bringing it to be repaired, or stealing it. The "object" of the act identifies the act in its most basic relationship to the moral law. If the act is chosen, its "object" is necessarily chosen. It can be chosen for different reasons and surrounded by different circumstances, but the person doing it cannot "jump over" the "object", because the "object" is precisely what he/she is doing.
The "circumstances" of an act are elements of the moral order which qualify an act already rendered good or bad by the object. Some circumstances will change the nature of an act, while others will render it better or worse. Stealing from a blind man aggravates the evil of the act of stealing. Circumstances are "who, when, where, and by what means".
The "end" of an action is the motive or purpose for which it is performed. Sometimes an action is performed for its own purpose. Here the "end" and "object" would coincide. I may drive through the mountains just for the sake of taking a drive. But an action can also be used to accomplish another purpose, in which case the action chosen is a means to an end. The "end" or goal can then render an otherwise good or indifferent act (according to its object) into an evil act. If I drive through the mountains in order to help a bank robber escape justice, my driving has become an evil act.
The end or motive cannot make an act whose object is evil into a good act. We can see this clearly in abortion itself. The object is the destruction of innocent life. No matter how good the motive or purpose may be (solving a social or financial problem, eliminating mental or physical distress, or even saving life), the object of the act remains unchanged and unmitigated in its evil.
Here we reach a crucial principle. The person performing the act cannot jump over or absorb the object (nature) of the act into its end or purpose. If I choose a means to an end, I necessarily intend the means. I cannot say I am choosing only the purpose, and hence ignore the evil of the means. This principle is behind Paul's assertion in Romans 3:8 that it is wrong to say, "Let us do evil that good may result."
It can be very tempting to absorb an evil act into a good motive or result. If I could save the whole world from being blown up by killing one baby, would I be morally justified to kill the baby? No. It is never justified. The morality of an act is not determined solely by its consequences. Consequences deal with what I bring about "on the outside", what changes I introduce in the world. But the action also determines me morally. The question I need to consider is not only, "What will my action accomplish?", but "What will I become by choosing it?" If I kill a baby to save the world, I have become a murderer.
The point that one who chooses a good end cannot thereby consider the evil means "not intended" is crucial in the determination of what means to use to end abortion. This principle does not in itself, of course, clarify which means are good or evil, but it does help us avoid a serious pitfall in moral reasoning. It helps clarify the difference between tolerating an evil consequence of an act (whose object is good or indifferent) and choosing an evil means to a good end. The difference here is the difference between doing good and doing evil.
An application to the specific case of shooting an abortionist causes us to ask, "Is defense of the unborn the object of the act, or a consequence which I choose as my goal and motive? Furthermore, is the death of the abortionist the means through which I achieve my good purpose, or is it an unintended consequence of a good act?" Note that if the goal is achieved by means of the harm inflicted, I cannot say that I did not intend the harm (or death). I cannot not will the means, I cannot not will the object of the act, jumping over it and calling the act by the name of the motive (defense). Any justification of harm or death inflicted as a result of an act of defense requires that I NOT intend that death or harm (and this includes that the harm not be the means to the end) but rather that such harm be an unintended consequence tolerated for the sake of a proportionate good that I do choose.
In summary of this rather technical point of moral reasoning, the evaluation of how much "force" or "violence" is justified in fighting abortion is not made by a mere "physical" analysis of acts ("photographing" the act, such as a person taking a TV out of a room). The (sometimes subtle) relationship of the elements of an act in itself (object, end, and circumstances) and the relationship of the elements of one act to another (means to end; act of killing or act of defense) need analysis in light of the principles that the intended destruction of human life is evil and that we may not do evil even to achieve great good.