A response probably consistent with police training. The wrong response for this kind of situation.
A more appropriate response would have been to shoot the aggressor till he stopped shooting others and do so without giving him a chance to shoot the responder first.
The correct way.
Bang bang bang bang bang..stop or I will shoot, bang bang bang.
On the other hand you inexperienced, untrained simple Simon, its not like the shooter was wearing mismatched military gear, a balaclava and firing an AK-47. He was wearing a US Army uniform like most or all of the others there and firing a sidearm the type of which the officer may not have recognized at first. For all you know, the officer may not have known if he was a bad guy or a good guy who happened to have the ability to defend the others and was trying to take down the bad guy.
Maybe telling the shooter to drop his weapons and identifying oneself as a police officer was more appropriate than you thought.
You weren't there and you don't have the whole story yet.
How would you like to be a citizen with a legally carried firearm who is trying to defend yourself and fellow citizens during a terrorist or criminal attack, and have the police shoot you down without giving you a chance to explain yourself?