To be completely honest, it is not unusual for someone to want to cause someone pain or harm within reason nor is it much of a disability especially when you are seeking revenge. Typically, disabilities relating to harm would involve actually going out and doing it due to mental incapability; however, most disabilities do not relate to violence which is a general misconception among the public. In fact, a 2014 study enacted by the American Psychological Association showed that mental illnesses are not directly linked to any sort of crime. They did diagnosed those with serious mental disorders and only 7.5 percent of crimes were of related symptoms towards that disorder.
Human empathy and compassion is different from person to person & has its limits. For some people, it may only extend to an individual's own social group. I am taking a sociology class for four and half months which is generally helpful for learning about these different social behaviors and groups so I wouldn't say that they lack empathy. Needless to say, morals and mores are also dependent on a particular social group and society.