Sure Muslims should not kill other Muslims, just as christians should not kill christians or jews should not kill jews.
PEOPLE should not kill other PEOPLE at all. Reality is they do, so your question should be why don't people just follow what they claim to believe. If they did we would live in a pretty peaceful world, filled with religious tolerence. Not perfect mind you because you will still have your rebels and anarchists and serial killers and just your average everyday insane person.
At the end of the day we are just human and we are not in heaven so we can only hope for the best and prepare for the worst. People will continue to die everyday defending themselves, their families, their beliefs and their values. This is not limited to Muslims, Islam, Turkey or Syria, its a universal epidemic.