There are many versions of Islam

One of the favorite activities of many Muslims of all persuasions is fighting of which kind of Muslims are the “real Muslims” and declaring those Muslims they disagree with to not be “real Muslims”.
The differences between many of these sects of Islam are often more stark than the differences between Christian denominations, which is one of the reasons Muslims often kill other Muslims for being “heretics”. Not understanding the fact that there are many different versions of Islam causes a lot of confusion among Muslims, non-Muslim religious people, nonbelievers (such as myself), critics of Islam and apologists for Islam alike.

http://www.real-islam.org/73_8.htm