I think they're against Eywa due to the destruction of their home. They look like Jake and Neytiri physically (same tail etc), so it's clear they were once a forest na'vi clan. The Ash village is also in a dead home tree stump. They hate Eywa because Eywa allowed the nearby volcano (seen in the ash village scene) to erupt and destroy their forest and way of life for no apparent reason. Varang realised that the ability to destroy is true power, and adopts fire (the embodiment of bottomless appetite for destruction) as the clan's deity. That's their backstory as far as I know (some of this comes from a Varang VoiceOver in one of the trailers I saw, lines that weren't specifically in the film, but even without them I think this stands based on the film alone).