A mental disorder characterized by heartlessness or a lack of emotions is known as "Psychopathy" or "Antisocial Personality Disorder" (ASPD).
Psychopathy is a personality disorder characterized by a persistent pattern of disregard for, and violation of, the rights of others. People with psychopathy often exhibit a lack of empathy and remorse, a manipulative and callous interpersonal style, and a disregard for social norms and rules. They may appear charming and superficially charming but lack a deep emotional connection with others.
It's important to note that psychopathy is a complex and controversial construct. Diagnosis and assessment of psychopathy require a thorough evaluation by a trained mental health professional. Additionally, not everyone who displays heartlessness or lack of emotions necessarily has psychopathy, as there can be other underlying factors contributing to such behaviors.
If you or someone you know is exhibiting concerning behaviors, it's essential to seek help from a qualified mental health professional for a comprehensive evaluation and appropriate support.