You probably heard about Valium (which is not an SSRI) being brought up because of the Las Vegas shooter. It is the most commonly prescribed psych drug, so if you think about it, Valium must be correlated with not performing shooting sprees, because otherwise we should see a lot more shooters having it as a prescription just by chance if there zero correlation. SSRIs as a category are probably the most common psych drugs overall, same logic applies even more so.