I tend to think that the Deviants come off as Evil because the Eternals are smug assholes, but just because there has never been a story about goody Deviants, it doesn't mean that there can't be. If there are millions (or billions?) of Deviants on Earth, they can't all be "evil" and lets not forget that there's cross pollination to consider like Maelstrom who was a Big Bad with both Inhuman and Deviant lineage.