It would seem that: No one who is in favor of capital punishment can be called pro-life, for "pro-life" means that one is in favor of protecting the life of all men, but being in favor of the use of capital punishment means that one is in favor of the death of some men. But the premises "no man should be killed" and "some men should be killed" are in contradictory opposition to each other, and thus if one is true, the other must be false; and conversely, if one is false, the other must be true. Therefore it is impossible to be both pro-life and in favor of the use of capital punishment.
Sed contra: The relationship between contradictory opposites follows from the principle of non-contradiction, for if both premises in a contradictory relationship were either true or false, then something would both be and not-be at the same time and in the same respect. But from this it must also be admitted that two premises are only in contradictory opposition if holding both would violate the principle of non-contradiction. And this is not necessarily the case with regards to those who are both pro-life and in favor the use of capital punishment. To wit, the premises are not held to both be true in the same respect.
For this to be properly understood, one must first consider the difference between the antecedent and the consequent will. As the Angelic Doctor explains when discussing how the divine will is always fulfilled against the objection that Scripture teaches that God wills all to be saved, yet Scripture and the holy Fathers also teach that not all will be saved (Summa theologiae, I, q. 19, a. 6 ad 1), their is a difference between the antecedent will and the consequent will. This difference is not in the will itself, but in respect to the things willed. That is, a thing may we good or evil in its primary sense and when considered absolutely, and yet after some additional qualities or attributes are consequently considered it may be that the contrary is true.
Absolutely speaking, antecedent to any circumstances, that a man live is good and that he be killed is evil. But consequently to certain qualities or attributes it may be good that some man be executed insofar as it is necessary for the preservation of the commonweal. Thus it may be said that it is possible to have a general willingness that all men live insofar as they are men, and yet will simply that particular men be executed, when this is a just and necessary punishment, and that capital punishment thus be kept legal for this purpose.
Thus it is obvious how one can be both pro-life and in favor of the use of capital punishment. To be pro-life is to have a general willingness that all men live and to will simply opposition to those act or species of acts that unjustly take human life. But to be in favor in favor of capital punishment is to will simply in favor of those acts that justly take human life. And insofar as this is done for the purpose of the protection of the commonweal it is consonant with the general willingness that all men live, since those guilty of capital crimes and justly executed are those who are a danger to the commonweal because they are, by their own choice, a danger to the lives of other citizens.