Lord Barkis Bittern from Corpse Bride after finding out that Victoria is poor after he married her. He murders Emily so that he can get her fortune and it is implied that he plans to do the same to Victoria. He also attempts to kill Victor.
Zira from The Lion King 2. She's MUCH less composed and self-controlled than her predecessor Scar.
The Hyenas from the previous film also qualify. They are known to crack jokes relating to eating their victims when they find prey, as evidenced by how they behaved in the previous film when they trapped Nala, Simba, and Zazu.
Ursula of The Little Mermaid. When she transforms into Vanessa, its implied that she lost quite a bit of sanity (to the point of becoming borderline Ax Crazy) when turning into her, as she talks to her mirror in a manner similar to a schizophrenic, emits a psychotic grin when throwing a pin at a mirror's head with enough velocity to knock the mirror back, and most certainly kill a person had that been a human being, and then her cackling. She's most certainly Ax Crazy when she has her Villainous Breakdown after Ariel destroys her eel pets though.
Actually its hinted that she was never sane to begin with...if you take notice of the fact that she has former mermaids as a polyp garden in the entrance to her cave...
Rumpelstiltskin. Don't be fooled by the trickster shell. Deep down, he's really a homicidal sociopath.
Prince Charming in the Scared Shrekless story "Boots Motel", (a parody of Psycho's Bates Motel) which Donkey and Puss in Boots make up.
Lots-o-Huggin' Bear in Toy Story 3. Although initially represents a relaxed personality, he, during the climax of the movie, hit the phone and Big Baby with the tip of his cane. In fact, he is so mentally unstable that even his subordinates fear him.