If we go on what others say of us, then we drift inexorably to spectral evidence that plagued the middle and dark ages and wound up putting eight million mostly innocent women to death for alleged witchcraft. Spectral, or hearsay evidence is not reliable. Unfortunately, we need something concrete based on fact, documentary evidence and physical evidence complete with continuity. We have to be aware of a number of factors that can contribute to the development of such incidents. These range from bullying, peer pressure, prejudice, economic circumstances and a genuine mental condition the predisposes someone to such actions. This makes the task of tracking such individuals difficult as most people are not trained to be psychiatrists or criminal investigators.
When re rely on hearsay, anyone can be implicated for anything. There is such a thing as fabricated evidence when someone is seeking vengeance or retribution. If you can't find something in a mudslinging campaign, then something can be invented. Many people rely on hearsay instead of asking the penetrating questions to weed that out. This incident could have been prevented if security screened for weapons in advance of the rally.