Too many guys shooting at him. They should have had one guy do it, and chosen someone who had better aim.
If time permitted, yes. However, when the first bullet flies, the reaction of cops as well as bad guys is to start shooting, too. It takes a LOT of training to slow down, think, and focus on putting your sights on the BG instead of focusing on the BG and just pulling the trigger. Unfortunately, most cops don't have that much realistic training.
I'm not up on my Shotgunology, but every one I've fired holds limited rounds and takes a second hand to ready the next shot (difficult if you're holding a hostage in a chokehold) - assuming this is correct, there probably was no need for multiple cops firing after he took that shot.