Multiple personality disorder; Disorder or defense mechanism

I have recently read about multiple personality disorder, and have began to wonder about this one qustion. Is it a disorder? or a defense mechanism. Most cases of multiple personality disorder develop from some trauma, such as the death fo a person, or being rapped at a young age. This is what leads to my question of, could this be a defense mechanism. This dose somewhat relate to Freud's theory of the superego, ego, and id. Perhaps this is the Superego's trump card. THe other side of it being a disorder, that a persons personality dose not work like it is supposed to, or something goes wrong in the brain.

I can tell you from experience, it's not really something you want to have. In my case, Most of the time I will be the most calm, collected, and loving person you could ever meet, but the second I see a girl being treated wrong by a guy, under any circumstances, my mind just feels like it snaps and I become very physically hostile wanting to just tear the guy apart in the worst way possible. this of course derived from a child hood experience.

I don't really see it as a disorder though, it's really just a conditioned stimulus that triggers a deep rooted conditioned response.