This cop committed multiple Federal felonies. Google up 18USC242.
I would have said something like, "Sure you can film! Just do it while you're walking away from this area, which is now a crime scene. Or did you want to be taken to the station house to be interviewed over then next several hours about what you saw here and your phone taken as evidence?"
The public wants all the bad guys off the streets and all the crimes solved. But they don't realize it's not an episode of Law & Order and bystanders get hurt all the time.
Again, let me say, this cop is an idiot. Don't accuse me of defending him.