Assuming those you've seen were actually ones that were engaging in violence and not simply your harmless garden-variety crazy, how does that number actually compare with the number of people who you see every day who you don't even notice because they're not violent? Cognitive biases are interesting things, and one that people commonly fall prey to is overestimating the occurrence of rare events because those events stick out in our minds while the far, far more common everyday occurrences do not. The prevalence of mass media only contributes to this perception bias as rare events are what are newsworthy, but as a result people only see news of these rare events and thus overestimate the frequency with which they're actually occurring. The picture people end up developing from all of this is of a world with dangers around every corner and thus a need to respond to every report of a potential threat like it's an actual threat because everyone knows how common these dangers are, right? When in reality it's tough to get much further from the truth.