Why do Sunnis and Shiites kill each other (besides the fact they're insane)?

Sunnis and Shiites dislike each other and often kill each other because of a trivial disagreement from several centuries ago. Their difference is about who are the legitimate successors of the uneducated moron Mohammed. These assholes should want to get along just fine because they're equally retarded.

What Is the Difference Between Sunni and Shiite Muslims--and Why Does It Matter?

The Sunni branch believes that the first four caliphs--Mohammed's successors--rightfully took his place as the leaders of Muslims. They recognize the heirs of the four caliphs as legitimate religious leaders.

Shiites, in contrast, believe that only the heirs of the fourth caliph, Ali, are the legitimate successors of Mohammed.