According to the Mayo Clinic, “a personality disorder is a type of mental illness in which you have trouble perceiving and relating to situations and to people — including yourself. There are many specific types of personality disorders. In general, having a personality disorder means you have a rigid and unhealthy pattern of thinking and behaving no matter what the situation. This leads to significant problems and limitations in relationships, social encounters, work and school.” People don’t always neatly fit into one or another of the personality disorders. Often there is an overlap of symptoms.
You tell me that your mother may have narcissistic personality disorder. It sounds to me like your mother also has a sadistic streak. The connection between narcissism and sadistic personality disorder has been explored in the research literature.
If a person is sadistic, it means that he or she derives gratification from punishing, harming, or abusing others. Most people don’t purposefully inflict physical and/or emotional pain on others because they have empathy for others (including animals) and a strong sense of morality. Not so the person with sadistic personality disorder.
Not everyone with narcissism also shows sadistic tendencies. Most narcissists have an inflated view of themselves, have difficulty getting along with others, and need to feel in control to feel safe. But they don’t get satisfaction from inflicting pain.
Narcissistic and sadistic people generally have low self-esteem. It is thought that both generally are the result of confusing negative to abusive experiences in early childhood that result in inferior feelings. To compensate, the person goes overboard and acts superior. In the case of sadists, one proof of superiority is the ability to inflict pain. It is unknown if there is also a biological factor that contributes to the behavior and thinking of those with personality disorders.
Engaging these people in treatment is difficult. From their point of view (since they believe themselves to be superior) any problems in relationships are the fault of the other person in the relationship. Your mother’s response is typical. She thinks what she is doing is funny. She isn’t able to empathize with the pain of the pets. She doesn’t understand why you don’t share her amusement. But, if by chance, she starts to understand that she has a role in why others are upset with her, there is treatment that has been found to be helpful. Often it includes medication and cognitive behavioral therapy.
I wish you well.