Differences in Beliefs
Just look at what sufferings have come upon humans out of differences in beliefs, ideologies, religions, and principles. Read the histories of religious massacres from the ancient times in Egypt, in China, in Assyria, in Chaldea, in Elam, and Iran. See what happened to the followers of Mani and Mazdak, how Israelites suffered, how Christians suffered during their first eras, and how they were fed to wild animals for amusement. See how devastating the Crusades were between Muslims and Christians, and how each side massacred thousands upon thousands and squandered the force and strength that should have been used in collaboration and cooperation for nothing.