Yardbirds & headshots.....
I disagree with training to take "headshots". That to me requires an advanced level skill set(on the level of SWAT/HRT/tier 1 spec ops). There are many, many problems with aiming at a human head unless you are at CQB distances(3-6'). The head is small & not very thick. It's not always going to work.
If the armed subject is wearing body armor or moving, I'd aim for the lower torso. It will disable the spree shooter & allow you to engage him/her then take any required follow-up shots if needed.
I'd add that skill training & use of force/spree shooters should include multiple bystanders or witnesses not involved in the event. Stress, chaos and manic behavior should be expected.
It's not PC, but I'd also say that you may expect that type of reaction in low income/poor areas. Some "bystanders" feel compelled to get directly involved in any event they see. As a security officer(armed & unarmed) I've seen it occur often. Police & patrol deputies deal with these crowds all the time at crime scenes. About a year ago, I went to the scene of a "shots fired" call at the low end hotel property where I worked. An unknown male fired a few shots then fled the scene. The parking lot was flooded with "yardbirds" who milled around causing a huge problem. I thought to myself how difficult a use of force event would be due to the ignorant bystanders in harm's way.
The US Army Rangers & spec ops troops had to deal with these situations constantly in Restore Hope. It's very stressful & annoying.