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?