Have you ever heard of the American Civil War? Or of the German Christians fighting the English Christians as in World Wars 1 and 2?
Christians kill each other over land. Americans kill each other over politics. Muslims kill Christians and Christians kill Muslims, Jews, Aboriginals and anyone else who gets in their way. Native North Americans and other Aboriginal tribes world-wide kill each other for food, territory, women. The ancient Jews killed off entire populations (if the Old Testament can be believed) in order to have land for themselves. History is full of humans killing each other for: women, sacred sites, land, food, money, revenge, the list goes on and on and on and on.
So why do Syrians kill each other? I think they might share that gene called Homo sapiens, like all the rest of us.