Wrathion is a member of a species that is incredibly long-lived, tremendously powerful and hyper-intelligent, among other features. To think that his morality or ethics would be like ours is probably a poor assumption. I may be giving Blizzard writers more credit than they deserve but I hope they recognize that about the character. Dragons live for hundreds, sometimes thousands of years (depending upon the fantasy setting). They're insanely powerful with alien thoughts and motivations. They would play long games that a human is not even capable of comprehending. They can pull off long cons that would span periods of time longer than the existence of entire civilizations. Their entire concept of chance and probability would be foreign to a human being because of the length of their lives. Events that would be likely to never happen in the span of a human's life would be a guaranteed occurrence in a dragon's.
In short: they idea of an dragon is completely alien to a human and his ethics and morality would probably be so as well. Is he shady? I think so. Does it mean he is "evil?" No, lots of shady people mean well. Humans, to a dragon, would be akin to what livestock and domestic pets are to humans. Does the dog or the cow ask if a human is "evil?"