Le Guin -- Language, Dreams and Gods

In The Word for World is Forest there is a very important connection between the Athshean word for dream also meaning root and their word for world also meaning forest. This is very showing of how dreaming lays the entire foundation for Athshean society, and how all the physical world, or "real-time", as a forest grows from the roots of their dreams. I think this makes sense in that the roots are the part of the forest we don't see, but although they are imperceptible they support the entirety of the forest.

So dreams, or ideas, are the foundation for all Athshean action in the real world. Thus, Selver's role as a god/translator means he has the power to translate between the two; he takes the visions in the dream time and makes them manifest as action in the real world, not just his own actions but the movement of his entire species. This is akin to a god being able to create something from nothing -- Selver brought about violent revolution in the real world from the concept of murder in the dream world.

Taking this symbolism even further, the killing dream is said to be an evil dream, so it is like a poisoned root of the forest. We can think of it as poison from Selver's questoining whether he got the idea of murder (the evil dream) from his own people, or from observing the actions of Davidson and humanity, whose dreams are likened to being poisoned (from when he talks about the state of dreams produced by drugs). So a poisoned root will cause the entire forest to rot from the core, meaning the concept of murder and killing will eventually bring about the fall or ruin of the Athshean forest -- their entire world, or civilization. Hence why Selver is so conflicted over killings and the feeling of guilt he has only now discovered, and why he would insist on ending the killings. For he also fears that, because dreams outlive the people who dream them, that this evil dream of murder that corrupted Athshean society from the terran influence will continue to poison their society and make irreversible changes to their way of life.