Their riches, possessions, land and even freedom was taken away from them, but they still have their music, their song. Music gives hope, and it allows the heart to speak much more than what can be said by just words. Our God understands this, and I believe He has a special heart for music as well. It is a very powerful thing.
The psalm ends quite violently, and I am sure people have used this line to show a very dark side of Christianity. But hold on, this is Christianity – real people with real issues dependent on a real God. The Israelites wished this much harm to their captors. If you have been wronged, you have experienced wishing that the person who wronged you receive the same level of pain you’ve experienced. It’s human nature. What would make the Israelites say such things? Unless they themselves have experienced such great pain and loss. If left on our own, humanity will be wiped out. We see it even now, when we are supposed to be wiser and put more value in human life. We need a Just God, we need a Merciful God, we need a Gracious God. A Powerful God, that can mold men’s hearts. Not by force, but by his amazing relentless grace.