A few centuries ago, the water avatar, Korra, failed to stop Vaatu from destroying the Raava, the avatar spirit. Korra did defeat Vaatu afterwards, but some people didn't believe that she actually did, and Vaatu still lies in it's spirit tree today. When Korra passed away, people who doubted Korra realized that the dark avatar spirit, Vaatu, was the only avatar spirit that was left. That would mean that the avatars would always be dark and evil. That's what everyone was taught, and it only led to one solution...