If you're going to be that loose regarding interpretation, you can make pretty much every character in Dragon Age "shady." And Dorian was working on time magic with Alexius before Alexius fell from his "good" Tevinter path. It was academic. And if people want to interpret researching something that someone else eventually used for ill or being in proximity to a slave owner as "shady" then they can, but they're idiots with a tenuous grasp on both morality and reality.
Let me state this again: I don't feel that way about Dorian. I know others that do, so I put him on the list. I agree with you completely. The slavery aspect doesn't bother me because I'm a history nerd. Tevinter is much like the late Roman Empire and I know that the inspiration for the nation comes from that time period.
In Rome, a slave could actually have a really good life. Oftentimes they were not just fed and clothed, but given a formal education as well. They could go on to be tutors, advisers, merchants and accountants on behalf of their masters. In fact, many times they were treated better than the average citizen. They could also buy their freedom as well, and become "freemen." Slightly lower in status than a citizen, but of higher rank than a slave (obviously.)
So I don't think Dorian is shady due to his views on slavery because of my knowledge of Roman slaves in RL history. He even points out that the alienages and the slums aren't really any better, and I agreed with him.