This is my explanation for the Kyuubi attack. You don't have to read it. But it makes sense to me.
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A cold wind blew through the branches, shaking leaves from their positions in the trees then buffeting them around until they landed in the ever growing piles of forest litter. Crisp autumn air, signalling the start of winter. The forests inhabitants had prepared their various burrows and dens, and had settled into the long sleep. A pack of wolves prowled, hunting for possible food. Deep in the forest there was a place no animal went. Even two leggers avoided it. A dark place filled with whispers and hints of things best left alone.
And from that place things came. Things of immeasurable power and strength. Things that could not be stopped by anything. They had used that place as a gate between this world and their own for millennia beyond knowledge, they came to study and mingle with the beings of this world. In their natural forms they were similar to the beasts with noticeable differences. But the animals and humans though pitifully weak and defenceless, were intelligent enough to recognize their power as a threat. And reacted the only way they knew how. With violence. The beings that came in peace were attacked and driven back to their own world. The humans gave them a name. Demons.
The 'demons' fled, not wishing to fight an opponent that was so mediocre. There was no honour in such a fight. It was putting a small dog against a dragon. But one of them returned to that world many years later, a young one. Too young to understand the danger. The humans saw it and attacked. First it fled, running for the way to its home. But its path took it close to a human dwelling. The village it neared was one with a higher form of human. Ones who generations ago had learned to utilise their inner energy from the first human-shaped 'demons' before they had begun to fear them. They gave themselves names for what they were. Shinobi. Kunoichi. Ninjas.
They sensed the power and attacked to protect the village. Frightened and hurt the young 'demon' used its own power to defend itself. Made itself bigger, more threatening. But the humans continued their attack. Driven mad by pain the 'demon', no more than a cub, stumbled and flailed, destroying parts of the village killing many of the humans. One of the ninjas, a leader sought to save his people. Using his life he sealed the 'demon' into the body of a human newborn.
Naruto Uzumaki.
Back in G'mer, the world of the 'demons', anarchy reigned. The cub that had ventured back to the forbidden world was one of unknown potential. They discovered what the humans had done. Many called for their deaths. Some requested a rescue. But it was decided by the elders. The gate was to be sealed; no one was to ever go there again. Humans were too intelligent. They advanced too quickly in terms of evolution, making them susceptible to grasp at the superior power of the G'meri. The gate was sealed and buried.
The only G'meri left in that world were ones that had hidden themselves. They were the mad ones, ones that liked to kill. They were either hiding or bound into humans. They would never have a way home. They were the demons.
What makes someone a demon? Their species? Or what they are inside? Some humans are more 'evil' than any G'mer.