How can a religion justify murder in the name of God?

I know this question has been asked before on Fluther, but hearing this story about the fresh wave of violence in Baghdad today between Sunnis and Shiites (86 dead), I find myself pondering once again how any religion, let alone different sects of one religion, can justify murder in the name of their God, any God. There are many questions I’ve all but given up trying to answer for myself because I’ve decided eitherthere is simply no answer or too many, but this one never stops haunting me. Serious thoughts?