https://www.nbcnews.com/news/us-news/connecting-mental-illness-gun-violence-misses-point-experts-say-n821696

According to a 2016 report published by the American Psychiatric Association (APA), just ensuring weapons don't enter the hands of the mentally ill isn't enough to prevent mass shootings from occurring regularly. Instead, the report suggests that public health and education campaigns are needed to teach people how to report concerning behavior to authorities and how to learn coping skills for anger and conflict resolution.

The report also proposes that policies and laws be focused on dangerous behavior that shows risk for committing gun violence, rather than a blanket category for the mentally ill, and recommends that institutions and communities develop threat assessment teams that can evaluate reports of potential danger.

As Fox asserts, the belief that the mentally ill are more likely to take part in a mass shooting appears to be a misleading. There were 198,760 homicides committed by a firearm in the United States between 1999 and 2015, according to the National Center for Health Statistic. Despite the high number, the APA report from 2016 says that fewer than 1 percent of firearm homicides are committed by a person diagnosed with a mental illness.

Metzl has researched the correlation between mental illness and gun violence, and he said that it’s a tenuous connection at best.

"There’s no mental illness diagnosis that explains causality," Metzl said. "There’s no mental illness whose symptoms are shooting anyone else. Most mental illnesses cause people to withdraw from society."

Some studies show the rate of violence among those diagnosed with mental illness is no higher than that of the population in general.

Yet, we use mental illness as a determining factor to prohibit that group from exercising a right.

There are many factors in a person's life that can influence their decision to turn to violence.

If you watch Criminal minds, it's obvious most people have some type of mental illness or behavioral issues to some degree.  Often there are stressors and a trigger event that sets someone off to be violent.  As such, everyone is susceptible.

That being the case, there will never be a predictable causal effect between violence and any mental or behavioral issues.  You can only analyze and identify them AFTER the violence has occurred. 

If only the violent ones were more considerate, like specifically announcing they are planning to hurt or kill people in mass numbers.

Like everything surrounding the gun violence question the issue is more complicated that most will bother to think. There is no single group "mentally ill"

You have different illnesses, some of which are not associated with violence or actually associated with a lower risk of violence. On top of that many if not most mental illnesses exist on a scale where they can be anywhere from mild to extreme.

Banning guns is treating a symptom but doing little to nothing to address the cause. We need to address issues on why people decide to kill large amounts of random people. The public also needs to recognize that these individuals represent a small portion of gun murders. I agree that more education on recognizing and coping with behaviors is a useful step towards this issue.