Slavery alone makes them a condemned society. Then there's blood magic and sacrifices, and all those treats lasts since the formation of the Imperium, not since MvT war.
Well, I wouldn’t want to live anywhere in Thedas: honestly, these people are all horrible, when it comes to civil rights 
When thinking about the way Thedas works (considering the not-quite-so-high fantasy setting it was meant to be), I couldn’t imagine my noble being completely fair to the elves. She sure was outraged at selling elves, but was sort of “yeah, it’s horribly bad but not a really pressing issue” when it came to the fact that they in fact are the second class citizens with almost no rights whatsoever. Status Quo is easy to accept, when you are in a relatively privileged position.
If Tevinter is anything like the late Roman Empire, lots of people would think that slavery is just another social institution and is, well, ok. I mean, most of Fereldens think that mages should be left to live all their lives in circles and elves are not real people (Shartan, what Shartan ?) and somehow they still perceive themselves (unlike those horrible Orlesians, decadence and all ) as fair and free people.