Muslims have been killing Muslims in the name of Islam since the beginning of Islam.
"Aisha bint Abu Bakr (died 678) (Arabic عائشة Transliteration: ʿāʾisha, [ʕaːʔɪʃæh] "she who lives", also transcribed as A'ishah, Ayesha, 'A'isha, Aishah, or 'Aisha) was one of Muhammad's wives. She had an important role in the early Islamic history, especially after Muhammad's death, in the time of third and forth caliphate. Aisha and Hasfa were two insubordinate figures among Muhammad's wives. She was a strict opponent of the third Caliphate, Uthman ibn Affan. Aisha believed Uthman had become an apostate due to neglecting the Islamic traditions during his caliphate, and she encouraged people to kill him. During the siege of Uthman, she was asked to stay in Medina, but she left the city without paying attention. In her absence, Uthman was assassinated and people chose Ali as Caliphate. Having an implacable hatred against Ali and his family, she formed a rebellion army including Talha and Zubair and went to the city of Basra to seek vengeance for Uthman's blood, which was the beginning of the second civil war in Islam. Upon entering the city, she ordered 600 Muslims beheaded, including 40 in the grand mosque, who were considered to be in Ali's side. In the war, Aisha accompanied the rebel army in her camel-litter. Ali overcame the rebells, and the defeated army was treated with generosity and Aisha was sent back to Medina. She died in 678 in Medina."