Now just to lay out the flip side of this:
The responding officers, at best, knew the perp had shot and killed someone. They had no way of knowing it was a targeted shooting and for all they knew, he was about to start shooting more people, not just at them. What may have been in the balance is how many people will he kill before he's stopped him later verses how many will we hit if we fire immediately.
Maybe they did or didn't go through that thought process, but they took the later course. 9 innocents were injured, but we'll never know how many lives they might have saved.
Not taking sides, just presenting another view.