Speaking for myself, I would say there is no reason to stand there and get murdered. If you can intervene without making things worse, go for it.
Cops do train for active shooters, not as much as I would like sadly, but we do train. Thats the key word right there "Active Shooter". Meaning we are trained to run towards the gun shots, where the active shooter is. I can't see cops mowing you down if you are not shooting people or presenting a threat. We are going to be looking for the guy shooting people.
I would hope the perfect storm of the cops arriving right as you engage the bad guy (so it appears as if you are the shooter), may end in disaster. I don't know of any way to ID yourself as the "good guy" and not the "bad guy". It's a tough call.