How can horrific actions be not only condoned, but encouraged and considered something to be praised in some places in the Bible?

Psalms 137:9 is saying that someone would be blessed if innocent children were to be violently slaughtered: 

"Blessed shall he be who takes your little ones 
and a dashes them against the rock!"

How can it be that this is considered acceptable, and even praiseworthy?

Psalms 137:9

ESV - ⁹ Blessed shall he be who takes your little ones and dashes them against the rock!

Clarify • Share • Report • Asked January 19 2017 • Samuel Bourassa