I believe that classical exegesis gives us the tools for unlocking even the most unfortunate parts of Scripture. Thus, we look upon the aforementioned verse, Psalm 71:13:
Let them be confounded and perish that are against my soul; let them be covered with shame and dishonour that seek to do me evil.
Then we scratch our heads and pray, thinking about what sort of spiritual meaning could lie there. And then John Cassian says:
When we read or sing all these things [of violence and hatred in the Psalms], therefore, and others like them that have been included in the Sacred Books, if we do not take them as having been written against those evil spirits that lie in wait for us day and night we shall not only not derive from them any increase of gentleness and patience but we shall even conceive a kind of cruel feeling that is contrary to gospel perfection. For we shall not only be taught not to pray for our enemies and not to love them, but we shall even be incited to detest them with an implacable hatred, to curse them, and unceasingly to pour out prayer against them. (Conf. 7.21.7-8, trans. Ramsey)
Who are against your soul? Who seek to do you evil? Evil spirits. Demons. The Psalm in the Christian’s hands becomes a tool of spiritual warfare to combat the forces of darkness.
Let us now, therefore, consider the most famous of the violent Psalms, Ps. 137. The Psalm closes with the following:
O daughter Babylon, you devastator!
Happy shall they be who pay you back
what you have done to us!
Happy shall they be who take your little ones
and dash them against the rock! (NRSV)
Typically, the Revised Common Lectionary avoids these passages, and if we consider how most of us read the Bible most of the time, I can understand it. However, if we look to the ancients, such as John Cassian or C.S. Lewis, we find new ways of looking at the above verses:
It behooves us as well to destroy the sinners in our bed — namely, our fleshly feelings — on the morning of their birth, as they emerge, and, while they are still young, to dash the children of Babylon against the rock. Unless they are killed at a very tender age they will, with our acquiescence, rise up to our harm as stronger adults, and they will certainly not be overcome without great pain and effort. (John Cassian, The Institutes 6.13.2, trans. Ramsey)
And C.S. Lewis:
I know things in the inner world which are like babies; the infantile beginnings of small indulgences, small resentments, which may one day become dipsomania or settled hatred, but which woo us and wheedle us with special pleadings and seem so tiny, so helpless that in resisting them we feel we are being cruel to animals. They begin whispering to us, “I don’t ask much, but”, or “I had at least hoped”, or “you owe yourself some consideration”. Against all such pretty infants (the dears have such winning ways) the advice of the Psalm is the best: knock the little bastards’ brains out. And “blessed” he who can, for it’s easier said than done. (C.S. Lewis, Reflections on the Psalms, pp. 383-384, in Selected Books)
And there you have it, the spiritual interpretation of the difficult Psalms at work.