http://en.wikipedia.org/wiki/2012_Aurora_shooting
Tucson: "Loughner had been arrested (but not convicted) once on a minor drug charge[11]" - drug addiction combined with mental illness is the predictor here. The mental illness alone was not predictive.
https://en.wikipedia.org/wiki/Jared_Lee_Loughner
> Acquaintances said that Loughner's personality had changed markedly in the years prior to the shooting, a period when he was also abusing alcohol and drugs.
Again: drug or alcohol addiction are reasonable predictors of violent behaviour. Mental illness alone is not a reasonable predictor of violence.
Mental illness is common. It would be unusual for there to be no mass murders with a mental illness. All you're doing is lazily ignoring all the very many murders committed by people who do not have a mental illnes o focus on a tiny minority committed by people who do have a mental illness; and then you're ignoring the actual predictors of violent behaviour which are previous violence or drug / alcohol addiction.
I drink tea because I have a cold. Not everyone who has a cold drinks tea, though. I'm guessing that mental illnesses might be a sensitive subject for you for some reason, but being overly politically correct isn't the answer to solving the social stigma. You can't deny that people who have trouble separating fiction from reality have a mental illness.
By linking mental illness to violence you stigmatize people with a mental health problem. This stigma makes people less able to seek help from professionals or friends or even family. People become irrationally scared of those with a mental illness and don't know how to offer help. Employers are less likely to offer employment to those with mental illness.
> I think most of us assume people who go on mass shooting sprees have some kind of mental instability.
But that's just ignorance caused by people who continue to lazily assume and then state that anyone who is violent is also mad - even if there's nothing to support that.
This is possibly an example of the conjunction fallacy. We see violence and cannot explain it, and so when we think "is this person violent, or violent and has a mental illness?" we end up at "violent and with a mental illness" even though it's less likely than "just violent".
http://en.wikipedia.org/wiki/Conjunction_fallacy
Linking mental illness to violent crime is as lazy and ignorant as Fox news linking violent crime to African Americans, with the same mangling of statistics and undue focus on very rare events and lack of focus on common events.
Here's some further reading with cites: http://depts.washington.edu/mhreport/facts_violence.php