No prayer for any revenge that would gratify the selfishness of our nature, a thing to be burned out of us by the fire of God, needs think to be heard. Be sure, when the Lord prayed his Father to forgive those who crucified him, he uttered his own wish and his Father’s will at once: God will never punish according to the abstract abomination of sin, as if men knew what they were doing. “Vengeance is mine,” he says: with a right understanding of it, we might as well pray for God’s vengeance as for his forgiveness, for that vengeance is to destroy the sin—to make the sinner abjure and hate it; nor is there any satisfaction in a vengeance that seeks or effects less. If nothing else will do, then hell-fire; if less will do, whatever brings repentance. Friends, if any prayers are offered against us because of some wrong you or I have done, God grant us his vengeance! Let us not think that we shall get off!

But perhaps, in saying “He will avenge them speedily,” the Lord was thinking of what most troubles his true disciples; and the suggestion is comforting to those whose foes are within them; for, if so, he recognizes the evils of self, against which we fight, not as parts of ourselves, but as our foes, on which he will avenge the true self that is at strife with them. And certainly no evil is, or ever could be, of the essential nature of the creature God made! The thing that is not good in us, however associated with our being, is against that being, not of it—is its enemy, on which we need to be avenged. When we fight, he will avenge.
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Commentary

Vengeance!

Few things are more troubling in Scripture than passages that deal with vengeance. The desire for revenge, as we commonly think of it, seems to appeal to all that is the worst in us. We don't want to bless our enemies, we want to get even! Like Jonah, we don't want to hear that our enemies are about to have a death-bed conversion!

However, I don't think any of us shed many tears over the demons who, cast out, fled into a herd of pigs and were, presumably, dashed to bits against the rocks when they ran off a cliff (animal lovers: the pigs themselves were quickly whisked into, well, pig heaven!). Jesus came to save us from sin, and MacDonald's meditation above helps to resolve the dissonance we sometimes feel when thinking about divine vengeance.  

Any consideration of this topic leads inevitably to consideration of the pleas for vengeance expressed in the imprecatory Psalms. 

from Psalm 137:

Remember, O Lord, against the Edomites
    the day of Jerusalem,
how they said, “Lay it bare, lay it bare,
    down to its foundations!”
O daughter of Babylon, doomed to be destroyed,
    blessed shall he be who repays you
    with what you have done to us!
Blessed shall he be who takes your little ones
    and dashes them against the rock!

For my money, the most insightful commentary on these passages comes from C.S. Lewis' Reflections on the Psalms. He notes that such passages convict us, as we realize they reveal "the same thing in [our] own heart[s]." But going deeper, Lewis observes that when we injure another, we commit two crimes: we've harmed our neighbor, and we've tempted him to feel the sort of raging resentment expressed above. Finally, however, Lewis points out that violent feelings against injustice are a sign of being closer to God, not further from Him, when compared to the "total moral indifference" to matters of right and wrong, good and evil. And he ends in much the same place as MacDonald: "For we can still see, in the worst of their maledictions, how these old poets were, in a sense, near to God...Not, of course, that God looks upon their enemies as they do:

He "desireth not the death of a sinner." But doubtless He has for the sin of those enemies just the implacable hostility which the poets express. Implacable? Yes, not to the sinner but to the sin. It will not be tolerated nor condoned, no treaty will be made with it..."