Role of Inquisitor and Inquisition in DA4

And? It is still possible to have a "successful thriving empire" and still be 'bad'. Just look at the Old Tevinter Imperium. It was, by every observable definition, successful and thriving. Magical and technological development, massive expansion, rising economic power, the building of wonders, and so on. But that still didn't erase the fact the empire relied on (and continues to rely) on slaves and their abuse. Egregious abuse. And when Andraste, Shartan, and the rest came with their overbearing armies and no doubt slaughtered noble families, their knights, and their servants, I am sure she would fall under the label of "terrorist".

Another example would be the Qunari. Their society is known for their technological advances, advanced standards of living, and a powerful military. That's a successful and thriving empire as well. But we all know there is a cost to that. Freedom is limited for everyone because every person is forced to perform their predetermined role or risk punishment (or re-education), and even they have their own slave class in the form of the quameked viddath-bas who they basically reduced to animals of burden. A terrorist taking down that society in the name of freeing those victims would no doubt be bloody and result in serious collateral damage. But you can certainly see why several people would join that cause and call it 'good'.

Even if we assume the evanuris were not overly malicious and mustache-twirling villains, that doesn't erase the heavy implications that Elvhenan had an underclass that was clearly unpleased with their situation and were willing to throw their lot in with Solas to change it. In fact I'd say its pretty much confirmed at this point. It likely wasn't limited to just the elves either, but the dwarves and possibly even humans. The only question now is just how badly they were treated, and evidence is pointing toward badly for a good chunk of them.

When Andruil has people praying "Spare us the moment we become Your prey" and Elgar'nan has a monument made in a single afternoon by a "thousand-thousand toiling servants" who were later explicitly referred to as "slaves" in the same codex entry....yeah. Unless we learn that these refer to criminal cannibals or something, it looks like their successful thriving empire had a few fundamental problems that likely leaned toward the morally reprehensible.