The more I look at it, the more I see that deep down, Solas is a reflection of the darker aspects of the positive points of some of your companions. Cassandra is all about faith. Solas has a deep and profound lack of faith. Cass believes that the Maker has a plan. She struggles with it, if you listen to her, but in the end, she grabs tight to her faith and doesn't let go. Solas...lost faith. In everything. He does it himself because he doesn't trust anyone else to do it. He doesn't deviate from his plan because he DOESN'T believe this world is better. He doesn't have faith, it shows in how he views the world. He thinks that if the 'magic' is brought back, the world will be better. He can't foresee a future without it. He doesn't trust the world to improve, to get better, so he believes he must burn it down, and rebuild what was once before.
Varric believes that you have to keep moving on. That you don't give up. Throughout the game, he continually strives to keep upbeat, even when life is basically beating him like Stewie beat Brian when he owned him money. Solas DID give up. He gave up on people. I don't think he's continuing on with his plan, he's falling back because he CAN'T keep going. He falls back on his plan to remake the world because he gives up on the now. Some part of him HAS to know that restoring the Elves of old is like replacing an angry dog with a much bigger, meaner far more rabid bear...and we ALL know how bears in Thedas work.
Blackwall has this rosy, idealistic view of the Wardens. Incorrect yes, but his view that they are heroes without measure is admirable. He gravitates towards it because he wants...he NEEDS to feel that he can be better than what he is. Solas lost his idealism. He is cynical, and sees heroes as chess pieces that will inevitably be sacrificed in a bloody game of war that he can never truly win. Solas needs to believe that something he has done can go right. So far, he sees himself as a failure. Every time he does something right, something worse happens. He tries to free his people, and he takes something from them. He tries to restore his people, he nearly dooms the world.
When he looks at Sera though, I've always though that he is mentally kicking himself. He tries a few times, to get her to think about the elves, to think about culture, and what he believes to be 'their' people. Sera doesn't care. She's moved past that. Solas...he looks at Sera, and he HATES himself for it. Why? Everything that she is, EVERYTHING...can be traced RIGHT BACK to what he did. To him, Sera is a constant, foul-mouthed reminder that he screwed up. He sees her and he thinks, 'I did that to the elves. She can't see what we were because I failed them.' Even a Dalish Inquisitor would make him feel such a massive build up of guilt that it would be nearly intolerable. A romanced Lavellan? If he had met her when the elves were immortal, he could have spent the rest of eternity with her. He doomed their relationship before it began because he is so set upon atoning for his mistakes that he refuses to see the future that he could have had with her. He gives up the NOW for the past. NOT the future. Sera and Lavellan are salt in the wound to him. In a way, he might even justify his actions by thinking that he is doing it FOR them. 'I will make the world so that future generations of elves never have to be so divorced from their People as I ruined the world for them'.
I used to think he was like Magneto...but, I'm not so sure anymore. He's more like, Xavier...and not just because the...er...hair...cut...no, it's because of who he is. A teacher, a mentor, and a friend who makes horrible decisions and is forced to continually fix them. How many people did Charles mind rape into submission? For the greater good? The inconsistencies in Cole MAY be attributed to Solas messing with his head to prevent others from knowing his plans. It's a wild shot in the dark, I know, but there it is. Thing is, what would Xavier have been like if HE had gone extremist and Erik had tried to be the hero? Well, they've gone that route in the comics, I think...ANYWAY...thing is, that is kinda how the Inquisitor and Fen'Harel evolved over DAI. Except, at the end, in this case, Xavier took away Erik's powers for good...