Without being there, or even having even seen what usually is a murky surveillance video that leaves out details and which itself is second-hand record of what went on, it may or may not have read as a certain crime to those who actually saw it. However, it may have - by dint of the youth of all, ("boys" not "men" was the term used here for those who were putting a hood over "a girl") - looked a bit confusing as to what was happening. That would be that for any intervention involving lethal action - for good reason: you don't endanger lives with an indeterminate scene in front of you, or even a doubt. License numbers, following the group outside while calling 911, those could be enough - and were for authorities - or someone - to determine this was a joke.
As to real crimes you mention, yes they are horrible. And maybe on a rare occasion a bystander can help. Practically speaking, there usually is little that can be done because of many things: the speed of the crime, an abduction, the fact that often there IS doubt as to what is happening: is the older man who grabs a 5 yr old and puts her in his car abducting her?, looks like it, but you only see the "middle scenes" of this story. You don't see the whole film: harried cell phone call of the man to his wife from the car saying "how could you lose her at a STORE!?" or him saying "O thank God" before he snatches his daughter up with high emotion and quickly puts her in the car, and speeds away home to make sure she's OK. Then calls his wife to tell her he found her in store parking lot and she seems OK so far.
Even bystanders who see what IS a crime, will often do the wrong thing. Case from the streets of NYC in point: psychotic homeless man acting violently squatting against wall on sidewalk, someone reports he has a knife: what do you do? pull your gun and fire, scream while pulling it: "drop the knife or I shoot", punch him, what? Here's what the cops did: not one cop, but 5 or 6 cars with 2 cops in each, 3 cars drive onto sidewalk forming wall around man, 2 other cars and cops hang back a few yards. Ambulance shows up. All cops leave cars casually and casually form an interior half-circle inside the perimeter of their parked cars, no hands are on guns, they are alert but relaxed. A Lieutenant takes position in middle of the half-circle and begins a long,quiet conversation with the perp, it is inaudible to any but the cops and the perp. After 10 min the talk seems to end and homeless man stands up on his own and quickly but quietly the Lt. cuffs the man with two officers helping, no rough handling of the man, the Lt. still quietly speaking to him while cuffing him. Two paramedics step in and all slowly walk towards ambulance. More quiet talk to the perp in the back of the ambulance, then slowly with flashing lights but no siren it drives away, followed by a squad car, no lights, no siren. It was perfect, professional, must have been done by these cops many times in the past, or rehearsed a lot. The crisis for the man was brought to a close by what seems a counter-intuitive means: non-violence. His acute crisis and danger to all was over. No cop, no bystander, no perp was hurt at all.
I don't think many bystanders in this, an actual threat, could have pulled this off, or even known to try it. Other and very confrontational ways may have produced a full psychotic, berzerk reaction, slashing knife, etc. with wounded and killed, including maybe the homeless and disturbed man.
So,either bystanders are not often in a position to "see the whole play" and really know if the bit of the scene they witness is a crime to begin with.
Or when it is a crime, their participation is based on lack of knowledge, lack of man-power, lack of proper tools to use. And they can unintentionally create more harm.
The few Bystanders that fall into neither of the above, yes they can help stop a killing perhaps. But this is one potential good next to an awful lot of chances at damage and harm.
