As a species, human beings love to argue, no matter what it’s about. Even if everybody agreed on religion, we would find something else to disagree about, whether that be the color of a certain dress or the acceptability of pineapple on pizza. In other words, it is not necessarily the fault of religion for being so divisive. It is the fault of people for being so easily-divided.
In the same vein, religion is often used as an excuse for violence, not necessarily the cause of it. People who bomb buildings or start wars in the name of God are just looking for a way to justify their violent tendencies.