"You have waylaid all his other plans. Now, as a petulant child, he will destroy the game board rather than admit defeat. Be ready for anything. He still believes himself a god, and gods do not fall gracefully."

We all presume (likely correctly) that Solas is referring to the Evanuris when he talks about Corytheus after you prompt him; how they acted, and how their actions would have destroyed the world.

What if he was actually referring to himself as the petulant child who destroyed the board instead of admitting defeat, not the Evanuris?

Solas himself described his 'younger self' as a hotblooded and cocky know-it-all. (And given he -presumably- got drunk and pissed magic in his past, that only adds to the not-so-great/immature former self). He wasn't really that great a person.

And then, take into account;

Elgar'nan, Falon'Din, Dirthamen, Andruil, Ghilan'nain, Sylaise, June.

7 Evanuris (the most powerful beings -at the time- in existence), all working against Fen'Harel. What chance did he really have against all 7 of them, especially after they killed Mythal?

Solas was losing the war/rebellion. And we assume he created the veil as a 'last resort' to seal the gods away to protect the world. What if he wasn't motivated by a sense of 'must protect people/the world' and it was more 'if I can't win, neither can you'? I don't think he really intended to survive severing the world's ties to the Fade.

We know from the story about Falon'Din that he spread war to garner more followers, to the point where all the other Evanuris had to step in, and 'bloodied him in his own temple' to get him to stop. It's not so far of a stretch to say they would do the same to Fen'Harel, given he's taking their slaves/giving them freedom and has been a general pain in their asses for ages.

They beat him in all of his plans, backed him into a corner. But he still had one last thing he could do; create the veil, and destroy their world (everything in the time of Arlathan ran on magic, and without that... It crumbled, it died - the board was destroyed).

And Fen'Harel was forever remembered as the god of betrayal, the reason the elves became enslaved and impoverished and mortal. The Dread Wolf fell disgracefully, remembered not for his wisdom, but for his petulance and his trickery.

And Solas had to deal with the consequences for thousands of years "in dark and dreaming sleep", unable to do anything to impact the 'real' world outside the Fade, except speak to dreamers/spirits and experience only memories of what's going on 'outside', knowing that everything he accomplished was torn down, and his people are now going extinct, all while he sleeps with nothing but whispers and his own thoughts - for thousands of years.

Imagine how much that would kill you inside, knowing that all of it was your fault, and those thousands of years of suffering and contemplation were your own doing? You can't even be mad about it, not at anybody but yourself.