I can't disagree Glenn with that and other similar responses in this thread. I do think there are some people in this world who are willing to act and also those that probably will not though - the same can be said for citizen rescuers in natural disasters, fires, and so-on.
http://www.policeone.com/off-duty/ar...ts-4-suspects/
I will also state that the off-duty officer in this story(copied and pasted from another recent thread on TFL) isn't Required to act in these situations(in my opinion).