I don't think he wants more power just because he's addicted to it. As I said I think Sinbad wants the same thing as Kouen, unite the world, or rather what he says in Sinbad no Bouken "change the world".He just got corrupted by this power. We also don't know exactly what happened between him and Al thamen , except for the fact that he lost someone dear to him because of them, and hurt Sindria in the process. He mentions what happened in Partevia, so I'm guessing it has something to do with the princess of Partevia, the one we see with Drakon.Maybe he loved her or something.
Anyway the key to understanding Sinbad and his motives is that event that has yet to be shown. And you're right about Mogamett's speech. I just hope he won't end up like Mogamett, because I don't think a miracle like him would fall completely into depravity.Plus according to Al thamen he's the second person they hate the rmost after Solomon, so I don't see Sinbad collaborating with them anytime soon.
Right now it's either him or Hakuryuu that will become the medium. My bet is on Hakuryuu, but who knows?.