Takfiri is a term for a Muslim who accuses others, including another Muslim, of apostasy. Hezbol ah considers members of ultra-hardline Sunni-dominated groups like al Qaeda and Islamic State to be takfiris.
Extremists seek to impose faith by fear and force within the House of Islam as well as on outsiders who resist bowing to Muslim wishes. One reasonably could suggest that there was silent collusion between so-called mainstream believers and extreme jihadists before things blew up in their faces. Extremists grew hydra-heads that turned inward inside the House of Islam.
The price of such collusion seems to have levied a heavy debt on the average believer who wanted only peace and security.