I'd say that an Evil sphere would be useful during god-gen; gods who represent everything their birth-culture considers evil (like Loki, who was a trickster, liar and oathbreaker in a culture where stealing was seen as worse than murder).
The Evil tag wouldn't show up on legends screen-rather, it would look at the cultural norms of the culture and nab a few spheres it didn't like (elves, for instance, might get a God of Evil with the spheres of Fire, Death and Tree-Cutting, whereas dwarves would get Deception, Cave-ins and Water). This would still allow other civs who didn't mind those spheres to later reinterpret them (for instance, Elves LIKE sneakiness, rain is vital for the plants, and cave-ins only punishes those who, like so many maggots, defile Mother Nature's holy flesh.).
Their birth-cultures might worship like some divine protection racket, rather than out of respect (give ten kittens on the altar to Ammu-Si, and she won't crush the mines this year, either, or flood them, and keep liars from your doorstep).
Gods without the Evil tag, but who have socially unacceptable spheres might be worshipped as protector deities against them.
Also, we need a system for generating bad-guys in the mythology, like Satan or the Titans or Jotun. Might be real megabeasts, might not be.