Peter Bergen and Paul Cruickshank ask why former al-Qaeda allies have turned against al-Qaeda's leaders:

To a large extent it is because al-Qaeda and its affiliates have increasingly adopted the doctrine of Takfir, by which they claim the right to decide who is a "true" Muslim. Al-Qaeda's critics know what results from this Takfiri view: first, the radicals deem some Muslims apostates; after that, the radicals start killing them. This fatal progression happened in both Algeria and Egypt in the 1990s. Its is now taking place even more dramatically in Iraq, where al-Qaeda's suicide bombers have killed more than 10,000 Iraqis, most of them targeted simply for being Shi'ite. Recently, al-Qaeda in Iraq has turned its fire on Sunnis who oppose its dictates, a fact not lost on the Islamic world's Sunni majority. LINK