Fenris may be biased, but that doesn't make what he's seen any less true- I'm sure Dorian has seen similar things. Also, being a slave would put Fenris in a position to see his master in a more true light and at his most depraved. I am sure there is some set of decorum and facade of morality in Tevinter. What is it we think Dorian is against if not abuses like Fenris has described?
Avernus did vile experiments on unwilling victims, how is that not evil?
On Dorian's reaction to Leiliana- I sensed guilt on feeling he was partially to blame for what happened given he knew the participants.
You know, morality is a very tricky thing. It's very subjective. I think I played that DLC, but I don't remember much, since it's been so long.
But, regardless, I'm not going to steer this topic into something that could very well lock it. >_> 
Anyways, I'm pretty sure someone said—I think Gaider—that Dorian didn't approve or was against all the corruption around Tevinter's politics and government, not necessarily the abuse of magic. But more so the way that people have a standing public figure, this perfection that they have to live up to and standards they must follow. They have to hide their entire private lives from the rest of the world to keep their position of power, as well as doing some pretty nasty things to get even higher up. I really think that's what bugged Dorian out; all the corruption and greed and bids for power stifling who he was as an individual and what he wanted to do.
I don't think it has anything to do with the abuse of magic. I mean, Dorian practices Necromancy, of all things, which is almost as bad as blood magic, as described by the Circle and the Chantry, so I don't see how it's okay for him to approve of the use of Necromancy and not Blood Magic. If not approve, I assume he'd be indifferent about it. It's magic; it's a tool. Why not use it if you want? Sure we've seen more people use it for wrong than right, but that doesn't mean there's not another use for it. All of my mages were blood mages (my Warden was also a Spirit Healer!), so I may be a bit biased because I also played very good aligned PCs and I intend to do the same with my Inquisitor (though Hawke was a snark and I'm probably going to do the same with my Inquisitor).
Also, you have to look at it this way. Dorian, even though he ran away from that life, still very much loves his homeland and its most of its core values, I think. He's had a taste of freedom—he's been free his entire life as a mage. No Circle, no Templars watching his every move, and the freedom to practice his magic as he damn well pleases. I don't see him wanting to ever take that away and I don't think he sees it as the cause of the whole issue with Tevinter's Magocracy. Whether it is or not the root of the problem, I think that's to be determined. It may have already been... I honestly think their use of magic and abuse of it is the cause for how corrupt it is, but we're talking about how Dorian sees it and I don't think Dorian sees it that way. He's an idealist, so he wants his cake and he wants to eat it too. That means freedom to practice any form of magic, but a society that isn't corrupted by power that has this idea of perfection looming over everyone. That's a very far fetched ideal, but a guy's gotta dream... right?
This is all my opinion, though, and how I see Dorian and his views. I may not agree with them and I actually think it's a little much, but I like stepping away from myself and looking at someone else's viewpoint. I love characters and how they think, especially the trickier ones and the ones that are more fleshed out. But, anyways, I honestly think we'll have to see because, right now, this is all speculation. I love speculation and it preoccupies me, but I really, really want to get to see for myself.
In other words, I want my Mustache Man right meow.
I can't wait any longer, Bioware.
