Khawarij
The Khawarij (collective plural Arabic: الخارجية, translit. al-Khārijiyyah; multiple plural: Arabic: خوارج, translit. Khawārij; singular Arabic: خارجي, translit. Khāriji; literally "those who went out") are members of a sect that appeared in the first century of Islam and has manifested throughout the centuries since. It deviated from mainstream Islam and was known for killing Muslims under allegations of takfir (excommunication). It broke into revolt against the authority of Caliph Ali ibn Abu Talib after he agreed to arbitration with his rival, Muawiyah I, to decide the succession to the Caliphate following the Battle of Siffin (657). A Khariji later assassinated Ali, and for hundreds of years, the Khawarij were a source of insurrection against the Caliphate. Descendants of the original Khawarij survive today in small numbers, in more moderate forms.
The Khawarij opposed arbitration as a means to choose a new ruler on the grounds that "judgement belongs to God alone". They considered arbitration a means for men to make decisions while the victor in a battle was determined by God. They believed that any Muslim -- even if not Quraysh or even Arab -- could be the leader of the community (imam) if he was morally irreproachable, but if the leader sinned, it was the duty of Muslims to oppose and depose him.