DiaboliKate--I think what you said in your first paragraph is very true, personally...it's my theory that he put on a mask of sorts and that over time, the mask became him. He did not want it to be seen who and what he really was, but he played the role so long that I think he destroyed in himself the capacity to be any other way.
Personally, I DID get the idea that there were self-esteem issues. MAJOR ones. He was always, always seeking validation from those around him, seemed to need it like food or air. He talked himself up, big time, yes...but when someone would deprive him of that external fix, he'd just go further and further and further until he got it, just like a junkie. What we saw--that massive ego...to me, it was more like a man on a high he's afraid to ever come down from, than anything genuine. And the one time he ever completely crashed--"Sacrifice of Angels"--it destroyed whatever was left of his sanity.