Most political commentators, and even Christian scholars, follow the advice of Thomas Hobbes, in Leviathan, Part I, Chapter XV. Rather than seeing an absolute prohibition on vengeance (Romans 12:17-21), they justify vengeance based on a pragmatic realpolitik:
A seventh [law of nature] is: that in revenges (that is, retribution of evil for evil), men look not at the greatness of the evil past, but the greatness of the good to follow. Whereby we are forbidden to inflict punishment with any other design than for correction of the offender, or direction of others. For this law is consequent to the next before it, that commandeth pardon upon security of the future time. Besides, revenge without respect to the example and profit to come is a triumph, or glorying in the hurt of another, tending to no end (for the end is always somewhat to come); and glorying to no end is vain-glory, and contrary to reason; and to hurt without reason tendeth to the introduction of war, which is against the law of nature, and is commonly styled by the name of cruelty.
It goes without saying that tempering the cruelty of the State with these (largely Christian) considerations is better than an unbridled dictatorship, but this language justifies the slaughter of millions of human beings.