That is a good point as well (and I would HOPE that the vast majority of pilots would do so!), but most pilots would take action as a last resort (or at least give the flight crew a chance to quell a situation). I think maybe that most pilots wouldn't take action unless they were certain that there was a "bad" situation going on, and watching in a rearview/through a peephole/waiting on some info would be the logical course of action. In the time it takes for a pilot to get enough info, make a decision and perform manuevers, it could be too late for Joe passenger beside Joe knife-wielder/bad guy.