I disagree to an extent. He's polite in Haven sure, but he's also distant and very controlled, one could say cold (and I don't just mean cus he had no shoes lol). Then you get moments from him like those throughout "All New Faded for Her", his drunk-on-intrigue attitude slip in the Winter Palace, his curt and decidedly less polite attitude with Morrigan in the Temple of Mythal, and his banter evolving his relationships with the rest of the inner circle and you see him basically thaw out. He was never a mean kind of cold, but there were walls and there was emotional distance and it lessens a lot over the course of the game.
I think considering him a megalomaniac is a step too far for the moment. I see where you are coming from with it, but I don't think he qualifies just yet. He may in the future, depending on his actions. But right now I think he's someone who is genuinely trying to do something necessary for the greater good, rather than his intent being to exert his ego/power over others. He can easily slip into the latter in the future, of course. It'll come down to whether he really cares about what's right or not in the end. If you show him another path, will he be humble/willing enough to take it? Or will he not, and show in the process that it really is about ego?
See, one of the main differences btwn Solas and Anders is that Solas says he wants to be proven wrong. He leaves that door open. By Act 3, Vengeance/Anders moved well past that, to the point where he says there is no way he can be wrong. There is hope for the former. There wasn't any for the latter in the end.
A defining trait of a megalomaniac.
Like Merrill - he believes he is a chosen one. That he is meant for a greater purpose to lead his people. This blinds him to any dangers that following his purpose can cause. That's pretty much textbook megalomaniac.
Anders wasn't a megalomaniac... he had a martyr complex.
@Xilizhra: That would totally end my interest in DA. What happens if they don't give you a way to do what he wants without an apocalypse? Is it a "too bad, it would have been nice to avoid but we're killing these millions of people anyway."?
EDIT:
@In Exile: I disagree about Solas and hurting people.
I think it makes him feel righteous to act like he doesn't want to kill people. To "struggle" with it even though his mind is made up. After all, if he struggles - how could he be a monster.
Real monsters struggle with the evil crap they do all the time... they still do it.