Interestingly, Pope St. Nicholas also refers to capital punishment in
Ad Consulta Vestra:
legacy.fordham.edu/halsall/basis/866nicholas-bulgar.asp
Chapter XVII.
Now then,
you have told us about how you received the Christian religion by divine clemency and made your entire people be baptized, and how these people, after they had been baptized,
rose up unanimously and fiercely against you, claiming that you had not given them a good law and also wishing to kill you and establish another king; and how you, having been readied against them with the help of divine power, conquered them from the greatest to the least and held them captives in your hands, and
how all the leaders and magnates along with every one of their children were slaughtered by the sword, though the mediocre and lesser persons suffered no evil. Now you desire to know whether you have contracted any sin on account of those who were deprived of their lives ** Clearly what you did not escape without sin nor could have happened without your fault, was that a child who was not privy to their parents’ plot nor is proven to have born arms against you, was slaughtered along with the guilty, although innocent**. For after the Psalmist said: I shall not go to my seat in the counsel of vanity and with people who do iniquitous deeds, I have hated the gatherings of the wicked and I shall not sit with the impious, [Ps. 25:4-5] he says a little while later in this regard, while praying to the Lord: Do not destroy my soul with the impious nor my life with the men of blood.[Ps. 25:9]…**You also should have acted with greater mildness concerning the parents who were captured, that is, [you should have] spared their lives for the love of the God **Who delivered them into your hands. For thus you might be able to say to God without hesitation in the Lord’s prayer: Forgive us our debts, as we forgive our debtors.[Mt. 6:12]
But you also could have saved those who died while fighting, but you did not permit them to live nor did you wish to save them, and in this you clearly did not act on good advice; for it is written: There shall be judgment without mercy for the person, who does not exercise mercy;[James 2:13] and through the abovementioned prophet the Lord says: Is it my will that the wicked man should die, sayeth the Lord God, and not that he be converted from his ways and may live?[Ez. 18:23] But because you erred more because of your zeal for the Christian religion and your ignorance than because of any other vice, with subsequent penance seek mercy and indulgence for these sins through the grace of Christ…
Chapter XXV.
You claim that it is part of the custom of your country that guards always stand on the alert between your country and the boundaries of others;
and if a slave or freeman [manages to] flee somehow through this watch, the guards are killed without hesitation because of this. Now then, you are asking us, what we think about this practice. One should look through the laws concerning this matter. Nevertheless,
far be it from your minds that you, who have acknowledged so pious a God and Lord, now judge so harshly, especially since it is more fitting that, just as hitherto you put people to death with ease, so from now on you should lead those whom you can not to death but to life. For the blessed apostle Paul, who was initially an abusive persecutor and breathed threats and slaughter against the disciples of the Lord,[cf. Acts 9:1] later sought mercy and, converted by a divine revelation, not only did not impose the death penalty on anyone but also wished to be anathema for the brethren [cf. Rom. 9:3] and was prepared to spend and be spent most willingly for the souls of the faithful.[cf. II Cor. 12:15] In the same way, after you have been called by the election of God and illuminated by his light,
you should no longer desire deaths but should without hesitation recall everyone to the life of the body as well as the soul, when any opportunity is found. [cf. Rom. 7:6] And just as Christ led you back from the eternal death in which you were gripped, to eternal life, so you yourself should attempt to save not only the innocent, but also the guilty from the end of death, according to the saying of the most wise Solomon: Save those, who are led to death; and do not cease freeing those who are brought to their destruction. [Prov. 24:11]
Chapter XL.
You say that it is a custom of your country that, **before you set out for battle, a most faithful and prudent man is sent by your lordship, who inspects all the arms, horses, and things which are necessary for battle; and if, at someone’s home, they are found to have been readied in a useless fashion, that person receives capital punishment: **now you wish to know what we think should be done in this case.
Truly we encourage you to turn all this [attention] to the arming of your spiritual weaponry and we advise you to turn the rigor of such great severity to the exercise of piety. For just as the preparation of arms and horses was hitherto investigated as to whether they were well suited to oppose the visible enemy, so now you should zealously inquire as to whether each person possesses their spiritual arms, i.e. good works, in readiness against the princes and the powers, against the worldly rulers of these shadows, against the spirits of iniquity in heaven.[Eph. 6:12]
You can’t help but notice! 
