Psalm 137:9

[chimp3]

I have to agree with GC on the meaning of the passage. It isn't an order to kill babies, it doesn't say you should dash babies against rocks. It's a lamentation, the psalmist is in such despair that the thought of their Babylonian captors suffering horrible fates is what he wants to sing about, and maybe it's also a prayer that the Babylonians be overthrown, or a prophecy that they will be.

It's a story of the response some Babylonians got when they demanded the psalmist sing for them.

1 By the rivers of Babylon,
there we sat and we wept
as we remembered Zion.
2 On the poplars within her we hung our lyres,
3 for it was there our captors asked us for words of song,
and our tormentors – for their amusement –
said, “Sing for us from a song of Zion.”
4 How can we sing a song of the Lord on foreign soil?
5 O Jerusalem, if I should forget you may my right hand wither.
6 May my tongue cleave to my palate
if I cease to remember you,
if I do not cause Jerusalem to be raised
to the very top of my joy.
7 Do you remember, O Lord, the Edomites on
the day of Jerusalem?
How they said, “Tear her down!
Down to her very foundation!”
8 O, Daughter of Babylon, you despoiler,
Happy is the one who pays you your recompense
as you dealt out to us.
9 Happy the one who will seize and dash your infants against the rock!