Dismissing mental illness as a contributing factor in gun violence in a thread whose aggressor was seemingly suffering from a mental illness is ridiculous regardless of whether or not you or I believe it is falsely attributed to other cases. "People blame X when it isn't true, therefore X is never true" is just faulty logic.
That said, I wouldn't exactly paint Dylann Roof, Robert Lewis Dear, Christopher Dorner, or James Eagan Holmes as mentally sound individuals. People who knew these individuals have come forward and described a number of concerning and shared traits, paranoia being chief among them. There is merit to the suggestion that undiagnosed mental illnesses are contributors in a lot of cases like these.
Read the damn article, it's not about dismissing it it's about understanding it and it's actual connection to violence.
Which you should want to instead of just drawing your own conclusions.