I have offered one explanation in this forum, and have been raked over the coals for it. Derek Flood's explanation is different. In the following article, he shows how Paul interprets various passages from the Psalms and elsewhere in a redemptive way, leaving out the violent portions.
Even Romans 12:19 where Paul quotes, "Vengeance is mine; I will repay" Paul uses it to instruct his readers NOT to seek vengeance, whereas the statement from Deuteronomy 32 indicates that the LORD will repay through the violence He brings about through His people. As Derek Flood put it:
If you would like to read Flood's article entitled "The Way of Peace and Grace," click the link below:In Romans 12:19-21, Paul again quotes Deuteronomy 32, citing the Lord’s declaration “it is mine to avenge” to argue that we should not seek vengeance, but rather work to “overcome evil with good.” In its original context, however, this passage was a celebration of vengeance: “It is mine to avenge; I will repay ... I will make my arrows drunk with blood, while my sword devours flesh: the blood of the slain and the captives, the heads of the enemy leaders.” This passage, which originally advocated vengeance and violence, is now used to promote enemy love.
http://sojo.net/magazine/2012/01/way-peace-and-grace