>If your logic were true they would not kill children or many
>other npc's, but they do. Red aura = maran bait.

Really, you have to accept that red aura is both psychic and omniscient. You can get it either for evil actions you have done or evil actions you are going to do. Infant duergar? The mystical red aura dispenser in the sky has the foreknowledge that this duergar is going to do evil things.

Sure, there could be a vanishingly rare duergar that will never do enough evil to earn a red aura. And when you find that special Duergar, you will know this, because he won't have a red aura. But if he does have the red aura, then you know he will, because the red aura dispenser is psychic.