Every time someone does something heinous, you tend to blame mental illness. The Colorado theater shooter, the Columbine shooters, Andrea Yates; all labeled "mentally ill." While there have been many crimes committed by people with mental illnesses, it is far from common.
When you label someone as mentally ill, whether they have been diagnosed by a qualified professional or not, you label the millions of law-abiding mentally ill people as criminals. I have Bipolar Disorder, but that doesn't mean I'm going to shoot up a bank. I have friends who have post-partum depression, but their kids are alive and well.
When reporting a story of a criminal, please refrain from labeling, or announcing a diagnosis. Those people with mental illnesses who commit crimes are a minority. Referring to a criminal as "mentally ill" only promotes fear and perpetuates stigmas. You don't say "The bank robber has had no diagnosis of mental illness," so why say that they have?
In closing, please refrain from using mental illness as a copout.
Millions of law-abiding citizens who just happen to have a mental illness