psalm 137 to which you refer is written from the point of view of the

captive.  the israelites in battle used to sing and shout praises to

God. Because of sin they now find themselves languishing beside the

rivers of Babylon and their captors mocking them require them to sing.

The irony is obvious.  The writer turns to prayer expressing his love

for his homeland and God and wishing the worst for his

captors.

The bad things that happen in this world is the result of sin. (The

wages of sin is death.)