Who gets to define Islam?

In a review of The Looming Tower: Al-Qaeda and the Road to 9/11, perhaps the best book to read about the origins of contemporary Islamic terrorism, Stephen Prothero explains that the real "clash of civilizations" is not between Islam and the West, but within Islam:

In a chapter on [Sayyid] Qutb called "The Martyr," Wright portrays Qutb (who was hanged in Egypt in 1966) as the progenitor of contemporary Islamist politics - for his claims that Islam is at war with modernity, that Muslims today must return to the pure Islam of Muhammad and his companions, and that all subsequent innovations to the tradition (including the Shiite and Sufi traditions) are but bastardizations of the one true faith. What is most chilling about Qutb's thought, however, is not so much his theological primitivism as his insistence that Muslims who disagree with him on any of these key points are apostates. His world is neatly divided into two warring parties - true Islam and barbarism - and any Muslim who disagrees with him is a barbarian. As Wright convincingly argues, it was this intellectual sleight of hand that "would open the door to terror."

To understand why this line of thought matters, it is important to note that the Qur'an plainly forbids the killing of other Muslims. As Wright discusses, the Qur'an does not shrink from war with idolaters. "Slay the idolaters wherever you find them," it reads, and "fight those who do not believe in God." Yet the Islamic tradition, including its four main schools of jurisprudence, also says that women and children must be spared in combat, and that Muslims must not target fellow Muslims for death. Commit the latter crime and you will spend eternity in hell.

In the World Trade Center, of course, many Muslims were murdered. The killings of Muslims in suicide bombings by al-Qaeda operatives and their imitators in Afghanistan and Iraq continue to mount. ....

...[T]he point this book really brought home is that what we are witnessing here is not so much a "clash of civilizations" between Islam and the West as a clash of Islamic civilizations - a culture war between, on the one hand, Muslims who believe that the Islamic tradition means what it says when it comes to killing women and children and Muslims and, on the other hand, those who don't - between Muslims who accept at face value the professions of faith of their fellow Muslims and those who excommunicate by imagination anyone who disagrees with their narrow interpretation of the one true faith. ....

Source: Who Gets to Define Islam? - Books & Culture