Who are Sufi Muslims and why do some extremists hate them?

The Islamic State targets Sufis because it believes that only a fundamentalist form of Sunni Islam is valid.

Some fundamentalists see the reverence for saints, which is common in Shiite Islam, as a form of idolatry, because in their view it shows devotion to something other than the worship of a singular God. Some consider Sufis to be takfiri, or apostate.

“The opponents of Sufism see the shrines and these living saints as idols,” Mr. Knysh explained. “Their existence and their worship violates the main principle of Islam, which is the uniqueness of God and the uniqueness of the object of worship.”