There's a mother I know of who didn't see that there was anything wrong with physically whipping her own son naked for at least 13 minutes every day during his kindergarten years. She wasn't even bothered by the state of the skin swellings she caused. She used cords and belts. This continued pretty much every day for the next 12 years until he finally moved to his father's home who abused him by using him as a "punching bag". He moved back to his mother's home a few years later but, although the physical abuse has stopped, she still shouts at him very often and clearly does not care how it makes him feel. I'm not looking for a word that describes a disease or disorder because she clearly knows what she's doing. She only acts this way towards her son.
She is sadistic
deriving pleasure from inflicting pain, suffering, or humiliation on others.
I suggest this 'son' find some professional help and guidance.
She may be described as:
I would recommend something like: Torturous, Maligning, Malodorous, Abuser, Psychopath, and more likely than a single word an adjective and noun pairing are going to best describe her.