My theory was that the Dark Avatar Spirit was the opposite side of the coin, that Aang's Avatar Spirit and the Dark Avatar spirit are two halves of the smae whole. Humans are not perfect, and the concept of good and evil is unique amongst humans. I believe the application of the original, whole Avatar Spirit forced a seperation upon the only line it could be split upon, shadow and light, evil and good, Yin and yang. The Avatar of Light (Aang's Avatar line) sought the good in people, whereas the Dark Avatar represented the worst of mankind. The time shortly after the beginning was that of destruction and despair (see "Cave of Two Lovers"). At some point, the Dark Avatar Spirit was removed from play, and the emerging nations lived in relative peace, with the Avatar maintaining the balance between them. Eventually, the pull of Yin and Yang to crash together and restore balance drew the Dark Avatar Spirit's prison back to the world in its spiraling path, bringing it ever close with each revolution, where it drew the curiousity of the Firelord, Sozen...

Yeah, except that Aang isn't a force of good, he HIMSELF is the balancer. He's only on the side of good because evil is winning the fight right now. I'm sure if there was too much light in the world, Aang would have to quash that as well. The Avatar represents a neutral balance, not good.

You can tell them apart by thier goatee.

And by the hats they wear.

And how they strangle puppies.

And by the different color arrows on their heads

NO, you can't. Their arrows are covered by thier hats. Obviously.

Yeah their hats stupid!