Khawaarij (الخوارج)

Historically, the Khawaarij are those who rebelled against ‘Ali ibn Abee Taalib (رضي االله عنه) in the 37th year of Hijrah (657 C.E.) and imputed kufr upon ‘Ali (رضي االله عنه) upon the arbitration that followed the Battle of Siffeen. They hated ‘Ali’s decision to end the fitnah which occurred with Mu’aawiyah (رضي االله عنه), calling ‘Ali’s acceptance of the arbitration a compromise. They also imputed kufr upon many of the sahaabah who accepted the arbitration. They did not even accept the whole of ‘Uthman’s (رضي االله عنه) Caliphate. This position stems from their misunderstanding of the Qur’aan. They deduced, wrongly, that major sins are kufr and thus forfeit salvation. Sin is a contradiction that nullifies faith! ‘Ali and the other sahaabah sinned in the eyes of the Khawaarij and that sufficed to nullify their state of belief! Their doctrine became famous in their rebellions against any Muslim leader who, in their eyes, committed a sin or a mistake. – [Dr. Saleh as-Saleh – Source]

Overview 

Others

Khawaarij makes Takfeer of the Musims due to Major Sins

Rebelling against the Muslim Rulers

Terrorism & Suicide Bombings

ISIS (Daesh)