How could the citizen do this without coming under fire from the cops? For all the cops knew, the cit could’ve been there to aid the perp. Either some info has been left out of the story and God bless the cit, or something has been left out of the story and the story smells.
OODA Loop. If an officer sees a plainclothes person providing cover fire to a fallen, uniformed officer, directing return fire against a determined, active shooter...
...it only takes seconds to ascertain the true threat.
Obviously every scenario is different and poses different risks, but this seems entirely plausible.