Some might argue that in sword and sorcery the supernatural is considered a violation, a transgression, and is therefore always evil. In the earliest sword and sorcery, the protagonists find themselves opposing the supernatural, fighting demons, slaying sorcerers. If elves show up, they are evil and deserve the blade. Accordingly, the barbarians and rogues who are the heroes of sword and sorcery stories are mundane and their enemies are extraordinary and fantastic.
This generalization does not hold for sword and sorcery any longer. Plenty of sword and sorcery stories and novels feature supernatural elements that are "good," that are on the protagonist's side. Nevertheless, in the earliest sword and sorcery, particularly Robert E. Howard's, the supernatural is indeed evil, is a transgression that needs to be corrected.
I think Howard and other early sword and sorcery writers's representation of the supernatural continue to influence the genre. So, in sword and sorcery proper, the supernatural tends to be marked by an aura of darkness. Although the supernatural might be on the side of the protagonist, the supernatural is always suspect.
We can really see this distinction if we compare the status of the supernatural in Tolkien with that of Howard. In Tolkien's "Middle-Earth," wizards can be both good and evil. Gandalf is an ally; Saruman is an enemy (Saruman is the perfect sword and sorcery bad guy, by the way). However, in Howard's "Hyborian Age," sorcerers are categorically evil. For example, consider the horrible Master of the Black Seers of Yimsha from Howard's, "People of the Black Circle," the archetypical evil sword and sorcery sorcerer. I leave you with one of the greatest scenes from this story:
"I think I will take your heart, Kerim Shah!"
He held out his hand as if to receive something, and the Turanian cried out sharply like a man in mortal agony. He reeled drunkenly, and then, with a splintering of bones, a rending of flesh and muscle and a snapping of mail links, his breast burst outward with a shower of blood, and through the ghastly aperture something red and dripping shot through the air into the Master's outstretched hand, as a bit of steel leaps to the magnet. The Turanian slumped to the floor and lay motionless, and the Master laughed and hurled the object to fall before Conan's feet--a still-quivering human heart.