Saintbarbido
Does the specifics of Raava or Vaatu really matter? Isn't the Avatar just the combination of a powerful spirit and human soul to help guide the elements and allow for the reincarnations to keep the memories and experiences of previous generations? It doesn't necessarily need to be one of those two just a powerful enough spirit. Even then the 'light' avatar wasn't a force of balance, it was a means of order and stagnation trying to have things not change. I'd liken them to the Jedi order that on the tin say they are a force for good but really just want the least amount of fighting within the world and hate change. Which is why I feel like Korra had such a hard time connecting to her avatar state until she started conforming more to her role as an avatar.