One of the central rules of Christianity is non-violence against anybody, believer or not (remember "turn the other cheek"?). That didn't prevent them to kill numerous Christians as well as non-Christians...

As for Islam, it started with the assassination of caliph Uthman in 656, shortly followed by the Battle of Bassorah, the Battle of Siffin and the Battle of Nahrawan, where tens of thousands of Muslims were killed by other Muslims...

Do you want me to list all the killings of believers by believers?

"People only accept change when they are faced with necessity, and only recognize necessity when a crisis is upon them." - Jean Monnet