Another Point of View
Another kind of lawful slaying belongs to the civil authorities, to whom is entrusted power of life and death, by the legal and judicious exercise of which they punish the guilty and protect the innocent. The just use of this power, far from involving the crime of murder, is an act of paramount obedience to this Commandment which prohibits murder. The end of the Commandment is the preservation and security of human life. Now the punishments inflicted by the civil authority which is the legitimate avenger of crime naturally tend to this end, since they give security to life by repressing outrage and violence. Hence these words of David: "In the morning I put to death all the wicked of the land, that I might cut off all the workers of iniquity from the city of the Lord." -Ps 100:8from the Catechism of the Council of Trent, the section of the fifth commandment. Translated into English by John A. McHugh, O.P., S.T.M., Litt.D. and Charles J. Callan, O.P., S.T.M., Litt.D.
"[T]he legitimate avenger of crime". Interesting turn of phrase, that.
(Personally, I'm not sure I would object to the abolition of capital punishment as a practical matter. As the late Joe Sobran pointed out, one has only to note who makes these life-or-death decisions. . . .)