The Kharijites considered the caliphate of Abu Bakr and Umar to be rightly guided but believed that Uthman ibn Affan had deviated from the path of justice and truth in the last days of his caliphate, and hence was liable to be killed or displaced.
The Kharijites thus deemed the arbitrators (Abu Musa al-Ashʿari and Amr Ibn Al-As), the leaders who appointed these arbitrators (Ali and Muʿāwiyah) and all those who agreed on the arbitration (all companions of Ali and Muʿāwiyah) as Kuffār (disbelievers), having breached the rules of the Qur'an.