I just got rid of a roommate who was exactly the way you described. She was creepy -- everyone tensed up when she walked in -- yet outwardly she was pretty, articulate, & put together. But very childlike. And mean -- she had rages & engaged in spiteful behavior if she couldn't get her way. She absolutely terrorized me & the other person living with us. Told out-and-out lies to the community (i.e. other girl pinched her all over). If not psychopathic, she might be borderline.
Apparently she had been abused by a schizophrenic mother (who later died in an institution) & raised by her father, who was somewhat otherworldly himself. He remarried (& divorced) someone who was also slightly off. Lethal combination: crazy mom, passive dad.
I strongly see the biogenetic base in these children. Just yesterday I had Sabbath dinner with some friends whose precocious five-year-old exhibited disturbingly hostile behavior. Kept attacking his baby brother. The parents, though very nice, were a bit off & probably unaware. The home was very messy & chaotic.
My former supervisor was like that. Outwardly sweet, a real southern belle, but an evil witch when she wanted to be. She would pit co-workers against each other and say contradictory things depending on who she was talking to. The worst was she would use things said in confidence against you. She reminded me of The Bad Seed girl or Single White Female. Really frightening to have to deal with.