The question of which religion has the most brutal history is difficult to answer because what criteria should be used to determine brutality? Should it be body count or the heartless and vicious treatment of others, beheadings videotaped and broadcasted on TV, or something else? It really doesn’t matter because every religion has had its brutal moments in history.
For example, Christianity had the Crusades and various inquisitions. Perhaps the Spanish Inquisition is the most notable, during which Jews and Muslims were tortured in unbelievably cruel ways. Ethnic cleansing by Muslims in the name of Islam has also been brutal to the Kashmiri Hindus.