In reality a shooter isn't going to let someone in his line of sight for long before actually shooting them. This isn't one of your video game fantasies.
But sure, let's have a bunch of additional armed civilians on sight and leave it to the police to wait and withhold judgement on if they are a good or bad guy until they see who they are firing at. I'm sure all the millions of police threads on this forum are firm believers in the competence of the police.