This is a man who awakens in a world long-changed from the one he is accustomed to. Where once magic was treated with awe and majesty, now those with potential are spurned and kept from achieving their true glory. The Tevinter Imperium is in shambles, crumbling from within and falling to decadence. The politicians there care little for bringing the Imperium's glory back and are content to grow fat as everything outside of their regal estates falls into dust. The magic they could use for so many wondrous things is curbed by fear and greed, and they are constantly in a war against the Qunari to the north.
This is a man who finds that the world teeters on the edge of destruction consistently by threat of the Darkspawn, monsters that bear the same corruption that he and the others found. Five of the Old Gods have perished (potentially) while the other two are silent and still, doing nothing to aid the people. Worse, his beliefs are considered heretical, so even if he wanted to worship them he would be unable to do so in most places.
This is a man who finds no record, no trace of the others he joined with to enter the Golden City. If he cared nothing about them, he wouldn't have bothered to look for them in the first place. But to find that his friends, his peers, are nowhere to be found.... it pains him, honestly. And so he endeavors to finish what they started, not just for himself but for them. All that they sacrificed has to have some meaning, and he wants to continue on for them.
It's not unlike Saix's comments on Axel's demise, where the former remarked that Axel was "weak" -- but anyone who knows the history between the two knows that Saix is truly in pain. On the surface, him saying the others are "lost" seems callous, but it masks a great deal of grief.
This is a man who is deeply afraid, of this new world, no matter how calm and assured he may seem. He will tell us to our face, if we say we are not afraid of him:
Words mortals often hurl at the darkness. Once they were mine. They are always lies.
He's telling us that when he breached the Fade to serve the Old Gods in person, he was afraid. He was afraid then and he's definitely afraid now (which I believe is partially why the Nightmare allied with him. Cory had his own fears that the Nightmare grew fat off of). How could he have not been afraid, stepping into the unknown, coming face to face with what he believed would be his gods on the other end? No matter his prowess, this was new territory for him, more so for the fact that Dumat had lost followers for some reason or another.
And what happens? He wakes up and calls out to Dumat, still loyal after all this time, and is seemingly met with answers in his fight with Hawke. And in my mind he no doubt returned to the Temple of Dumat and found it empty, abandoned, in ruin and lost of all its splendor. It's quiet, but he performs the rites that he did ages ago, calling out to Dumat for answers. Where had everything gone wrong? Why was the City black with corruption and dead whispers? Where were the others, why had Dumat forsaken his people? Why had they all forsaken them?
I believe this went on for a few weeks minimum and with an ever-increasing amount of silence given to him, he loses faith in Dumat. He believes that Dumat and the other Old Gods are lies, just as the Maker is a lie. Society begs for answers from deities they never see, in a world that grows worse and worse with each day. And thus he resolves to give the world certainty where there is none. A visible god that interacts with his subjects, who brings the world back to order. The Darkspawn would be taken care of, the "Old Gods" killed, Tevinter standing as the champion of the world with a rekindling of all the fires it once brought. Civilization would stand anew.
Some might say that he is a horrid person for what he planned to do to Calpernia. I would not contest that what he was planning was atrocious. His actions there are awful, just as his corruption of the Templar Army is (though for such corruption to have taken root, it had to exist independent of what he did in the first place). He planned to bind her to his will, effectively making her a slave (although how much the final one differed from the Erastenes project I dunno, but I have my thoughts).
But the important thing to take note of is that he recognizes what he is doing is awful, that what he will do to her is not a good thing. He hopes for her forgiveness, because he's trying to free her from the shackles of Mythal by binding her to him. It's not a good plan in that it's one form of slavery for another, but if he cared so little about her he would never have thought of the notion of him forgiving her for what he was going to do.
It bothers me that the narrative makes Corypheus out to be a caricature ("LOL arrogance personified!!!") with very little screen time and that also his depth of being that we could see if we were invested enough just from Legacy isn't touched upon enough. It makes me sad that the writers didn't see Corypheus' true depth. I looked at Corypheus and thought of what he would be like in the contemporary world when we first met him in Legacy, but Inquisition just... lightly scratches at it, in a quest you can only see if you make certain choices.
Characterization for a central character should never be so scarce or tied to choices in the narrative. This was the problem DAII had, and that Inquisition repeated it is sad.