Syiah doesn't have their faith to three of the four khaleefa after prophet saw died. They only believe in Saidina Ali r.a. They kill sunni because in their faith, it is not wrong to kill who is not have the same thought with them. I'm sorry, I can't explain it in very detail way like you asked for it but I have an article. Unfortunately it's in Bahasa Melayu. I can give the link if you understand malay.