I agree. There are solid arguments to be made on both sides when it comes to topics such as "higher standards", Qualified Immunity, and such. If you remember back a few years, there was the undercover officer who was shot and killed by a uniformed UCF (?) officer at a football game. The details are fuzzy in this old brain, but I remember at the time thinking how this was a preventable tragedy if the uniformed officer had made sure that the UC officer knew he was there, and had given him time to identify himself or ground his weapon and wait to be identified. I wasn't there, so...
Same here. I haven't seen the video, if there is any, but perhaps this could have been avoided if the responding officer had made his presence known, and held his fire until and unless the victim in this incident (yes, he's a victim) had displayed hostile intent.
Friendly fire, as they say, isn't.
As for me, I do a lot of "what if" thinking about getting caught in similar circumstances. What if, while off-duty and in civilian attire, I stumble up on a deadly force situation. How do I (a) intervene if need be, and (b) survive the first wave of responding uniforms? For my part, I've pretty much decided to (a) only draw if I intend to fire; (b) only fire if there is a lethal threat to me or anyone in my immediate vicinity; (c) scan, cover, and then get low and stay there until the cavalry arrives. They can disarm the bad guy and render medical aid after they cuff me. It's not worth getting shot trying to disarm or do CPR on someone I just had to shoot.
Maybe not the right answer, but it's the best I've got.