>>108301647In ATLA, the two worlds were "separate" like layered liquids of differing densities. It was possible, depending on the time and location, to swim between them. Despite two differing worlds, spirits, nature, and humans were inexorably entwined - for example, why would Hei Bai care about the ruination of human forests if he was solely a denizen of the spirit world? There were some outliers like Koh and Wan Shi Tong, but for the most part spirits were genial as long as the planet was respected. Remember, the greatest illusion is the illusion of separation.
In Legend of Korra, spirits are demonic hell creatures bent on the total ruination of humanity. Even the act of feeding your children is an unforgivable transgression to them. They take pleasure in human death and torture - twisting men into horrid abominations, or dooming them to an eternity of wandering in a fog where they eventually go insane. Rather than being a natural reflection of our world, a part of the whole, they swarmed into it from their own like an invasive parasite. Humans were trapped living on lion turtles, hunted and persecuted on their own planet.
Even Wan, murderer and spirit sympathizer, had to banish them back to their demented realm because clearly peace with them was impossible. The Spirits didn't even protest, implying that they could have always remained in their world, they just enjoyed slaughtering humans for sport.
