My wife being pregnant and not her usual, spritely self, I would be much more cautious about getting involved in Tinner666's scenario. In the past, when observing things getting heated, I have cell phoned 911 to suggest an officer might need backup, but I would not approach an officer I did not know unless his straits seemed acutely dire - and even then it would be with caution. I have had a weapon or two aimed my way by LE before, until the situation was verified, and I can't say I enjoyed it.
So, I am willing to help, but the level of help I will offer will vary with circumstances, which include the LEO's perceived level of need, and the feasibility of assisting.
That said, in an active shooter scenario, my first priority is security of loved ones. After that is assured, if I can stop a mad dog, and LE is not there, I will do what I can.
On a similar note, I have assisted at a couple of home fires, but backed off as soon as the shiny red trucks arrived - except for pointing out to the firefighters and paramedics where the origin appeared to be, in one case, and that an old man was likely in the house in the other (he was; they got him out; I kept his dog while he recovered from smoke inhalation over the next several months).
Got out of the way at a vehicle rollover, once the ambulance arrived, too - but my first aid kit had come in handy. A nurse who had also stopped was able to use it to good effect. A Coastie who had also stopped, and thankfully spoke Spanish, was able to help calm the injured kids and their mother. But we all got out of the way when the cavalry arrived.
Personally, that is how I think things should work.