I suspect we can argue principles all day, but if we are ever faced with this situation it will happen so quickly we will act completely instinctively. Your instinct might be to intervene or not. You might not even be able to understand the situation fast enough to act, even if your instinct is then to do so (too late).

Certain professionals are trained to recognise, assess and act on a situation in real time as it develops. Most of us do not have the benefit of that training. Whether we act the hero or the coward in the heat of the moment will probably have little to do with our ethical viewpoint.

Rob