Good point. Society conditions us to mistake class and refinement for honor. Chang may have been well-spoken and quoted Shakespeare, but he was still a liar and a traitor. Even by Klingon standards he was dishonorable, because he hid behind hired assassins rather than challenging and fighting Gorkon openly.
Chang reminds me somewhat of the various villains played by Sir Christopher Lee in that he's exceptionally charismatic- even charming- when dealing with others, be they allies, opponents, or otherwise. It's easy to give him more credit than he deserves because he carries himself so well, but his outward appearance is really just a smokescreen to mask his duplicity and treachery.
It's fortunate that Kirk and company foiled his plans at Khitomer. I've no doubt he'd eventually have taken the Klingon throne away from Azetbur and run the Empire into the ground in a bloody war with the UFP.