From the introduction:
There is hardly an area of biblical theology more troublesome to the Christian conscience than the so-called imprecatory psalms - psalms that declare a desire for God's just vengeance to fall upon enemies. They naturally evoke a reaction of revulsion in Christians schooled in the "law of Christ." [see the letter of the lady in the previous post]
...Since the character of God does not change, the essence of his ethical requirements does not change. Therefore, as the imprecatory psalms were at times appropriate on the lips of believers before the Incarnation, so they - or their like - are at times appropriate on the lips of believers today. There is a time and place to call for tangible, temporal divine judgment; there is, indeed, a "time to curse."
If [God] is to save his people from sin, oppression, and injustice, he must exact vengeance upon his enemies - the enemies of his people...Yahweh is a God who saves his people; but without God's vengeance against his enemies, there can be no salvation for his people.
Day helpfully gives four reasons why it's right for Christians to cry out for vengeance and violence:
1. "the vengeance appealed for is not personally enacted. Rather, God is called upon to be the Avenger."
2. "this appeal is based upon the covenant promises of God"
3. "both testaments record examples of God's people on earth calling down curses or crying for vengeance"
4. "Scripture further records an instance in which God's people in heaven, where there is no sin, cry out for divine vengeance and are comforted by the assurance of its impending enactment (Rev. 6:9-11)"
Instead of throwing out these songs of violence, as some have tried to do (yes, Isaac Watts, I'm looking at you), it is when we understand them Biblically and have a passion for God's glory and His church that we can sing them while worshipping in spirit and truth.