They are possibly the most shocking verses in the Bible.

Most of Psalm 137 is a lament by exiles longing for their homeland. 'By the rivers of Babylon we sat and wept when we remembered Zion...' They have been taken far away and they have been cut off from their emotional and spiritual roots. 'How can we sing the songs of the Lord while in a foreign land?' It's very moving, even with Boney M's 1978 'By the rivers of Babylon' as an earworm accompaniment.

But the last two verses, 8 and 9, say this: 'O daughter of Babylon, doomed to destruction, happy is he who repays you for what you have done to us – he who seizes your infants and dashes them against the rocks.'

That 'happy is he' is the same as the 'blessed are' in the Sermon on the Mount: 'Blessed are the meek,' 'blessed are the merciful'.

Yes: inspired Scripture is blessing the murder of children.

And that's a problem for preachers, because killing children is wrong – and the fact that it's in a Psalm doesn't make it right.

How, then, are we to understand a verse like that in a way that acknowledges the truth of Scripture and the wrongness of what the Psalmist says? Here are three ideas.

1. Pay attention to the stresses

I mean the way the emphasis falls when reading the sentences aloud. They fall on 'you' and 'your'. There's nothing benevolent about the Babylonians addressed at the end of the Psalm. They took Jewish babies and dashed their brains out on rocks if they thought they wouldn't survive the journey into exile. This Psalm isn't just a sentimental longing for a lost home, it's an outpouring of rage at everything that's been done to the Jewish people. So, happy is 'he who seizes your infants...'

2. Hear what isn't being said

Underlying the outrage in the Psalm is a deep sense that the world has a moral structure. It is not just that the writers and singers miss their homes and families. It is that what has been done to them is wrong. It offends against everything they know. Children should not be killed, and those who kill them are wicked. Even in these appalling verses, the Psalmist appears to distance himself – even if only fractionally – from the desire to take personal revenge: it is to be delivered impersonally, by a person or persons unknown, as an act of God rather than by his own hand. Taken at face value, this is an incitement to murder. Look under the surface, and it's a denunciation of sin.

3. Be creative with the text

This can be risky, especially if it means we ignore or skate over its plain meaning. But the Psalms, as CS Lewis pointed out, lend themselves to 'second meanings' of the kind the early church Fathers were very familiar with. He suggests of this Psalm: 'I know things in the inner world which are like babies; the infantile beginnings of small indulgences, small resentments... 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.' And, he continues: 'Against all such pretty infants (the dears have such winning ways) the advice of the Psalm is the best. Knock the little b*****ds brains out.'

Understanding these 'cursing' Psalms means learning to read the Bible intelligently. There isn't always a word-for-word correspondence between what it says and spiritual reality. They aren't designed – as the Letter to the Romans is designed, for instance – to teach doctrine, but to show how God's people lived in the light of a relationship with him. There are shocking things in them. There are shocking things in our lives, too – but everything needs to be tested against the standard of the gospel.

Mark Woods is the author of Does the Bible really say that? Challenging our assumptions in the light of Scripture (Lion, £8.99). Follow him on Twitter: @RevMarkWoods