Tell that to the 56 killed in the Peshawar mosque, tell that to the hundred of thousands of Shias killed by Isis in Syria.
Tell this to the hazaras in Afghanistan.
If you want to see Muslim, then see the large majority of them. one person can kill many in modern world by using bomb or assault rifle.
In each group there is always some tugs
Syria is different, that is war and both Shia and Sunni killied
This is why the country is destroyed, action create reaction, if you do big sin then God will respond, whether on earth or afterlife