IS members are jihadists who adhere to an extreme interpretation of Sunni Islam and consider themselves the only true believers. They hold that the rest of the world is made up of unbelievers who seek to destroy Islam, justifying attacks against other Muslims and non-Muslims alike.
Beheadings, crucifixions and mass shootings have been used to terrorise their enemies. IS members have justified such atrocities by citing the Koran and Hadith, but Muslims have denounced them.
Even al-Qaeda leader Ayman al-Zawahiri, who disavowed IS in February 2014 over its actions in Syria, warned Zarqawi in 2005 that such brutality loses "Muslim hearts and minds".