Assalam o alikum
Shia and Sunni never fight. There is a small group between shia's names nusehree's and a small group between sunni's names kharji. They fight with each other. Nusehree's are those who call Hazrat Ali (as) Aallah. And kharji's are those who thought that they know Islam better than any one. Made their own laws such as..... If any one commits a sin he must be slaughtered. These Nusehree's abuse Sahaba karam(rz) and Kharji's abuse hazrat Ali(as). These are the two groups which fight with each other not shia's and sunni's.