First of all, he's not talking about the Death Knight quest entitled, "How to Win Friends and Influence Enemies." Bartle does realize that Death Knights, while under Arthas's control, were horribly evil and he understand that this quest is necessary to make them look as evil as they are. That quest makes a very strong point that resonates with the storyline of the game.
He is quite aware that Warcraft involves killing, fireballs, destruction, and more killing. He knows there's a "dismiss quest" button at the bottom of the quest window. He is also aware that the game is not real life, it's only a computer game, and does not contain the Geneva Convention. So then, what's all the fuss?
Bartle's point is that The Art of Persuasion requires both Alliance and Horde members to test their own game written morality. The Alliance sees themselves as the good people of the game who stand up for the righteous and all that is holy while the Horde is embracing the concept of reformation and repentance, attempting to get away from the evil that was the Fel Horde. This quest should send up a red flag in the character's mind, but the quest text did not even attempt to bring those problems into focus.
In a world of cartoon-violence and evil personified in the context of "evil" characters, Blizzard's quest is too cut and dry for the moral problems it creates. Simply creating a torture quest that goes off without a scratch, provides reliable information, raises your Kirin Tor reputation, and lets you go on your merry way is too simple for characters who believe themselves to be so righteous. Bartle believes that shocking content needs to be labeled "shocking" by the game itself -- a self-referential nod that lets the player know that Blizzard understands that "torture is a complex issue."
Bartle's full response can be found on his blog, and it responds to as many of the comments made across blogs as possible while simultaneously clarifying his own position.