To put it another way: who is more "rightheous" the pally that fights for a good cause never letting as much as drop of evil taint his mind? Or the warlock who fully accepts that evil into himself and controls it to make it server the greater good? Could you trust a slightly corrupt pally (Arthas suggests "HELL NO" btw)? Could you trust a slightly corrupt warlock (basically yes, since we are all slightly corrupt to a degree)?
And to quote Batman TDK: Warlocks are not heroes WoW needs, but the heroes its deserves.