inb4 angry female Lavellans...
There is the possibility that the pre-Veil world was a very dangerous one. I mean Spirits and Demons were everywhere. So were Dragons and Titans. That is not a very safe place. The best analogy I can give is the Avatar universe. Before Avatar Wan, the first Avatar, Humans lived on giant turtles because the world was populated with Spirits.
So it is possible that group of Elves decided to establish settlements in remote corners in the forests of ancient Thedas. These leaders then became Evanuris. Initially they had good intentions but they got drunk with their power.
See, there's me trying to put forth a viewpoint that might come from an Evanuris that perhaps regret their actions that is different from the pre-Veil utopia that Solas claims to exist.
Err... Solas doesn't claim that the pre-Veil world was an utopia (our world is not an utopia either, but sure as hell it would look like one if we found ourselves on post-apocalyptic Earth). On the contrary, he compares Elvenhan to Tevinter.
Dorian: Solas, for what it's worth, I'm sorry.
Dorian: The elven city of Arlathan sounds like a magical place, and for my ancestors to have destroyed it...
Solas: Dorian... hush.
Solas: Empires rise and fall. Arlathan was no more "innocent" than your own Tevinter in its time.
Solas: Your nostalgia for the ancient elves, however romanticized, is pointless.
Solas: If you wish to make amends for past transgressions, free the slaves of all races who live in Tevinter today.
Dorian: I... don't know that I can do that.
Solas: Then how sorry are you?
Of course, that doesn't change a very real possibility that the Evanuris was once a force created for a greater and generally just cause. I mean, it's not like DAI was pounding into our heads just how many (all?) organizations were created and aimed to accomplish something good, only got corrupted in the end, ey

?
In fact, if you listen to Solas and his banters, especially with Cassandra or Sera after their personal quests you can get a feeling that he was indeed once an integral part of Evanuris, possibly going beyond Mythal's guard dog, as evidence in Trespasser suggests us.
They, in fact, may be a reason why he's generally not fond of organizations, or were a source of his first (or biggest, or at least very significant) betrayal, when it turned out that they've dropped any pretenses that they're on top for some sort of greater good, but for their personal ambitions, or what they thought world should look like.
It may also be the reason why he feels such overwhelming guilt and need to fix past mistakes - and the mistake might not even be the Veil per se. Oh sure, the Veil is a source of his woes now, but the reason that he was forced to create it may as well have been the necessity to protect the world from the very people he helped in their rise to power and - like Cassandra in case of Seekers - did so much he believed was good in their name, only for his trust to be used and broken.