it's actually kind of sad when you think about it

He didn't even know who he was himself

There's very interesting book about him - The minds of Billy Milligan by Daniel Keyes, you should read it. He actually was able to draw different types of art, and some of his personalities knew different languages and were experts in science, medicine, electronics, can play drums, sax, harmonica... It's unbelievable, how one man can do and know so many things at his age. (22 by the way)

My theory: multiple possesions

Sounds a lot like Sybil, in the way that each personality made her into a seperate person (hair, speech, mannerisms, some even wore glasses, one was an infant and another an old lady)

That's pretty cool, imagine him talking and all of a sudden he switches to a British accent

Yet he was still considered a criminal.. :/

There's actually research that shows dissociative identity disorder could possibly be entirely false