I hope you understand I am not excusing this horrific incident or any other violent crime when I say this:
There is a difference between someone being "crazy" or "unstable" in the colloquial sense and someone having a diagnosable mental illness. It is not a differentiation we make in general life because it's obvious to any thinking person that there is something seriously wrong with someone who can do these things. But the "wrong" is not to do with the function of the brain, in most cases. It is functioning as it should. Sometimes (most times) a murderer is just a murderer. And that is awful and doesn't make any sense. It feels so wrong, but it's true.
And when we use words like "unstable" to describe a violent criminal when it has not been reported that he had a diagnosed mental illness (at least so far, and that may change of course), the schizophrenic down the street who has never in his life committed a violent act (and is statistically more likely to have been a victim than a perpetrator of violent crime) feels less safe to go and seek treatment, or less motivated to continue his med schedule.
It's hard to worry about language at a time like this, when 50 people are dead. But times like this are when the media reinforces the stigma over and over again.