Vigilantism is tarring and feathering someone without trial or lynching them. Gathering evidence of wrongdoing by authorities is every citizen’s right and DUTY. The evidence is then given to the victim, who can choose to seek redress from the courts. Being a witness is working within the system, not being a vigilante.

Your bomber example is nonsense, unless you want to also ban cell phones. Someone could follow the bomber, call and say, “success” or “epic fail.”