I have two ideas, their not backed up by the series in anyway, or use evidence of them but if you care to read here goes. THIS MIGHT CONTAIN WRONG INFO AND I AM NOT SATANIC.

1. Shaidar Haran is not the DO himself, but rather his human form, when i say human i don't necessarily mean human. I think the Dark One to huge and inconsevible to imagine by the human mind, or picture. (Now this part i don't know if its right) The belief, there's God and there's Satan. Good and Evil. The many terms or many depicians of Satan or the devil have been things such as a man with goat legs, tail, and horns, or a man named Lucifier dressed in black, who walks the earth. Lucifier is not actually said to be Satan himself but a form seeable by man, human, who is able to walk the earth, and deceive people. While Satan himself is this evil evil unconsevible being who's just evil or the root of evil itself. Without going on and on, I believe that Shaidar Haran is like the man or creature form of the DO who is able to walk the earth, direct his 13 demons(Chosen), while trapped in the Bore. I may be completly wrong or full of s*** here.

2. I think that Shaidar Haran is a former human, but NOT ONE OF THE FORSAKEN. He was once a good guy from the AOL, a hero maybe. Aginor might have done something to him as a cruel experiment, maybe he's the first Myddrall ever created and he is immortal as the Chosen are, but he isn't or never was one of them That could explain his emotions of amusement that other Myddrall lack and his anxiousness to sever his tie from the DO completly. amusement, and his anxiousness to sever the tie between himself and the DO, i believe that he might have some good left in himself his human side.