People worried about him. They averted their eyes and took a long path around in order to avoid him. Who knows if he was dangerous or armed? The media loves to portray people with mental illness as dangerous. People without mental illness kill each other every day for money or drugs or jealousy or turf wars, but that isn’t newsworthy stuff because it is understandable, dismissible. It’s the murders that people don’t understand that make the news, and immediately get the “must have a mental illness” tag to them in an effort to give reason to tragedy, and give us a way to defend ourselves by avoiding people with mental illness.
Statistically, the highest proportion of violent crimes in the U.S. are committed by men under the influence of alcohol or other substances, but people regularly go to bars and other places where alcohol will be imbibed and then they avoid the person with mental illness out of fear of violence. That’s partially the effects of media skew.
I have heard some people say that being able to perpetrate a violent crime is the definition of mental illness. It would be nice to think that they are the exception rather than the rule, but it ignores a lot of history and cultural processes. Think about how many people are raped or murdered every day for the sake of religious wars, territory conflicts, genocide because of their race or ethnicity. The definition of mental illness according to the American Psychological Association specifically excludes someone from being classified as having a mental illness if their behavior is considered acceptable according to their cultural standards. For example, Native American shamans may hear the voices of spirits and talk back to them. This does not mean they have schizophrenia because hearing voices is consistent with the beliefs of the culture. Similarly, if a man is raised within a culture where raping a woman is considered proof of their virility and manhood, it may be against the law, but it is not considered a mental illness.
Those that think they can write a villain that acts cruelly because the villain has a mental illness are not taking the time to really understand the villain or their mental illness, and that can leave them flat and unbelievable. You can get away with it if you are writing stories on the level of sophistication as fairy tales for children, but most readers want to have some understanding of the villain’s motivation, and “because they are crazy” is flimsy writing. Even children’s books these days are trying to give the villain’s perspective. Many books and movies are taking to reimagining old stories to give the villains more depth, from Wicked to Maleficent. Even the most enduring comic books give villains reasons for what they do, such as the many origin stories of Batman’s villains. Yes, the Joker is crazy, but even he is given origin stories regarding what pushed him over the edge to villainous insanity. (Actually, the Joker has been given many backstories…).
The hero may not know why the villain is behaving that way, but the writer always should. Motivation for temporary villainous behavior can come from substances or the heat of the moment, but motivation for a pattern of villainous behavior should run deeper. It can be tied to the way the villain grew up, or particular traumatic events in the villain’s history. It may involve current unpleasant events like being blackmailed into doing unsavory acts. The villain may have cultural beliefs that support the acts of violence, or have other reasons he/she developed a distorted view of the world.
Mental illness can be tied in with any of these deeper reasons. It may not even be the villain’s mental illness originally. It can be close emotional ties with someone who had mental illness, such as how growing up in a home with domestic violence making it more likely for someone to perpetrate the same abusive patterns when they become adults. Mental illness may have shaped the person’s distorted world views, beliefs, and values. It can be close emotional ties to someone with mental illness that can cause delusions and secondary mental illness, such as how Harley Quinn falls prey to The Joker’s madness (This is called shared psychotic disorder, or Folie à deux.) To have a convincing and deep villain with mental illness, the author needs to know not just about the mental illness, but how it has shaped and been shaped by the person’s history, relationships, employment, world views, beliefs, and values.