Here is the whole of Psalm 137 from the NIV
That last verse is rather disturbing indeed. Those are the kind of lyrics I would expect from a band like Cannibal Corpse, not from the book of psalms.
1 By the rivers of Babylon we sat and wept
when we remembered Zion.
2 There on the poplars
we hung our harps,
3 for there our captors asked us for songs,
our tormentors demanded songs of joy;
they said, "Sing us one of the songs of Zion!"
4 How can we sing the songs of the LORD
while in a foreign land?
5 If I forget you, O Jerusalem,
may my right hand forget its skill .
6 May my tongue cling to the roof of my mouth
if I do not remember you,
if I do not consider Jerusalem
my highest joy.
7 Remember, O LORD, what the Edomites did
on the day Jerusalem fell.
"Tear it down," they cried,
"tear it down to its foundations!"
8 O Daughter of Babylon, doomed to destruction,
happy is he who repays you
for what you have done to us-
9 he who seizes your infants
and dashes them against the rocks.
Anyway, here is a video of the song.