Solas was following his own agenda throughout. My biggest disappointment in DAI was how little lore he actually told us after what we had been led to expect. I was thinking he was going to be the one to reveal lost history of the elves as seen through the Veil but instead I got irrelevant stories about old barbarian hordes, spirits and demons that were mildly interesting but not what I was really hoping for from him. However, since every time he let something slip about himself that I picked him up on because it seemed odd, I just got the standard "Solas disapproves." What he reveals about his actions in Trespasser fit perfectly with what we had been party to.
He thought to play Cory for a fool and let him activate the orb, the explosion then killing Cory, except Solas hadn't anticipated that Cory would survive. You see, Solas, that is why we have Grey Wardens. They figured out he couldn't be killed and that is why they imprisoned him. They got a bit lax in their later years but back then, they were on the ball. Solas probably originally went to the Inquisition in the hope of finding his orb. If he visited the site, he probably expected to find it there and then when he didn't, he went to Haven and offered to help, probably hoping to find it there. It was only after he saw the mark that he must have realised that his plan had gone horribly wrong. He couldn't confront Cory direct because he was still weak from his sleep, so he had no alternative but to offer his aid in the hope of recovering his orb. Remember his words when talking in Fade Haven about when the Herald ran up, Solas thrust their hand against the rift and close it "everything changed". He never does explain what he meant by that. If you choose the heart icon, you foolishly assume some romantic reference. I never did think of it like that but with my male Dalish he was just evasive in answering. Without the distraction of his romance I became far more suspicious of his motives, even without the scene with Flemeth, because of how little he ever did reveal of himself.
Solas maintains he is not a monster simply because he is going to allow everyone a few years of relative peace before he destroys them. Actually he is no better than Corypheus. Their reasons might be different but what they planned to do with the orb would ultimately have the same result. Also both of them took an action in the past that had catastrophic results for the rest of the world. Cory and pals releasing the blight on the world, Solas destroying his People's reality and ending their immortality. Neither apparently learned from their previous mistake. Cory had the excuse that he was an arrogant, insane, wanabe god. Solas is actually worse because he thinks he is better for feeling bad about what he is going to do and that he is being kind in giving everyone a temporary reprieve. He makes a great villain precisely because he seems so reasonable but I do find the idea that you can somehow redeem him by "proving him wrong" is laughable. If the evidence of history and everything you have done up to now hasn't convinced him to change his mind, what on earth could you do in the future that would do so?