There are some here who have a serious case of the armchair quarterback syndrome. It's really easy with 20/20 hindsight to judge someone else's mistakes.
I don't want Solas to devastate Thedas, but I can sympathize with his predicament.
First, his own people put themselves in a shitty situation, at the hands of power-mad "gods" and their whims. Oceans of blood flow. Thousands, maybe millions, are in servitude. The only "god" with any sense is murdered by the other "gods". All this combined with an immortal race spells eons of suffering and bloodshed.
I can easily see a game where we're dropped in as the "hero" to get ourselves out of a situation like that.
Or do you just set aside, knowing you have the power to *try* and change things, but you don't because you might fail? Or things may not turn out okay? That seems like the spot Solas was in before he put up the Veil. He was a powerful mage, yes. But he was up against a group of powerful mages. He could free slaves, but this powerful group of mages could use their propaganda as "gods" to scare slaves back into servitude.
Solas undoubtedly found himself in a desperate situation. He wanted, he needed, to break the bonds of servitude and free his people. He knew he didn't have all the answers. He knew he was taking a chance. He knew he was going to be weakened when he put up the Veil. But he wanted to have faith in his people, that they may use their freedom wisely and work things out.
We don't know everything that was happening then, but knowing what we know right now, I'd probably do the same thing Solas did. I would do just about whatever it took to free the elves from the influence of the Evanuris.
Then he wakes up thousands of years later and finds out that not only have his people been adversely affected, but other races have taken advantage of the situation and subjugated his people.
Once again he finds himself the only one with the power and know-how to help his people. Plus, he now owns responsibility for elven mortality and the strange effects the Veil has with the real world and the Fade.
So, he's not going to have a lot of sympathy for the people who have treated his people like poop. It's just not going to happen. Plus, all of his people are only a fragment of what they used to be. Their "natural state" has been tweaked so badly they no longer even know who they are or were.
So no, I buy no arguments that Solas is insane, or a megalomaniac or a psychopath.
In a perfect world, he'd have more empathy for all living things. But he's been hardened by war and other crappy situations.
That's why the role of the Inquisitor becomes vital in relation to Solas. It re-awakens some empathy in him.
It will be nice if we get the chance to find some kind of compromise and work with Solas. The Veil is an artificial construct. It's going down, one way or another. It's causing problems between spirits and the living. And the Evanuris... they need to pay for murdering Mythal and for the suffering they inflicted on the elves.