system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5-What caused the separation of Sunni and Shiite Muslims, and why is there violence?

Pertaining to the violence, I admit I do not know much about Islam, however I was under the impression that killing a fellow Muslim was forbidden. Surely wouldn't the most militant and fundamentalist branch spare their fellow Muslim brothers and sister?
gpt
After Muhammad died, Islam was governed by the Caliphs. The first four in order were: 

Abu Bakr ibn Qhuhafah,

Umar ibn al-Khattab

Uthman ibn Affan

Ali ibn Abi Talib 

Shiite Muslims view the election of Abu Bakr as invalid and view the first three Caliphs as usurpers, considering Ali to be the rightful successor of Muhammad, where Sunnis believe that the first four Caliphs were rightly appointed. As time went on, Sunni and Shiite diverged on more matters but that is the origin: "Who was the right successor to Muhammad"

As for the violence, some extremist Muslims will only accept someone else is Muslim if they practice Islam in **exactly** the same way - Otherwise, that person is an *apostate* (someone who has turned away from Islam) and is the worst thing a person can be.