In the Judeo-Christian tradition there are people of Divine Insight who do evil, such as the Diviner Balaam.
I'm wondering if there are Greco-Roman examples like this, especially regarding the Sibyls?
The most famous one is the necromancer Erictho, who originated in Lucan's epic Pharsalia (about the civil war between Caesar and Pompey).
In book five, Appius Claudius goes to the oracle at Delphi hoping to find out how the war will end; Apollo possesses the oracle's body and gives a prediction through her. In book six, on the other hand, Pompey's son Sextus wants something different—he seeks out Erictho, the infamous necromancer, and asks her to give him a better prophecy. Instead of praying and deferring to the gods, she threatens them, and gets results: she pulls a ghost back into the world and forces it into a recently-vacated corpse, making it use the corpse's mouth to describe what it saw in the Underworld (where time isn't linear).
About half the book is devoted to describing how monstrous Erictho is, in too much detail to copy here—but she definitely fits the "evil necromancer" trope, and is clearly meant as a foil to the Oracle at Delphi earlier in the epic.