American society tends to hold a structured and inflexible view of who is normal and who isn't; who is "just acting out" and may be able to function in society without assistance, and who needs to be locked up. So where lies the distinction?
Mental health is rarely black and white. The mind is so complex and intricate a phenomenon, it's a wonder anyone attempts to define a social norm.
Society is not kind to its outliers: it frowns upon on those who, voluntarily or not, deviate from its generic standards of behavior. Some people choose to defy mainstream society with regard to fashion or musical preference. Others have no choice in the matter because they are emotionally or mentally unstable as a result of problems including chemical imbalances in the brain and abusive upbringings. Mental illnesses are a key factor in the nature/nurture debate: are people born killers, or pushed into becoming them?
There may be genes that dictate personality disorders, but environment cannot be disregarded. Some theories conjecture that serial killer tendencies can be prevented from a young age with the help of a supportive home and attentive caregiver. Some deviants, though, can be helped through intervention so that they don't find themselves left alone and struggling to understand a world that kicked them when they were down.
The problem is that while sociopaths and psychopaths often deviate from the path of acceptable behavior to the extent that they may become dangerous, society over-generalizes and assumes that all "crazy" people need to be locked away. To do so is ineffectual, especially since they are placed into prisons with "normal" people who made bad choices.
For some people, mental illness can be controlled but not cured with medication, therapy or some combination of the two. People in this scenario still have the potential to be at least semi-functional members of society; all they need is a little help and support.
In many tragic cases, though, these people don't get the help they need. They feel like they can't speak about their inability to manage their emotions and thoughts. They feel isolated, ignored and "wrong." In more extreme cases involving personality disorders and other more severe mental illnesses, these people are not only lacking basic assistance, they often get kicked out of their homes – their only chance to benefit from any support system at all.
There are people missing the capabilities that make functioning in society possible, the people so lost in their own minds that they live in a different world that operates on a different set of rules. There are other people who just need someone who will listen. Society needs to become more tolerant of everyone and provide people in need with appropriate, humane attention, no matter their level of instability.