Realistically, a huge majority of the times when a private citizen uses a gun in self-defense, they never have to fire a single shot. Sometimes they don't even have to draw the gun, just be confident and move as if they might have a gun. That's the reality.
It's also true that the majority of people who defend themselves successfully by shooting never had any training at all.
People with training are better prepared to cope with the rare extremes than people without training. And it's the rare extremes we're talking about here: shots at a distant, moving target that is surrounded by innocents in an overwhelming, quickly-changing environment.
For example: Nick Meli did the right thing. He's a smart man and yes, a hero, for trying to save lives at the Clackamas Mall, but also for knowing and respecting his own limits. He'd be more of a hero if he could have dropped that scumbag. Perhaps he could have done so if he had known that he could go down to one knee to change the angle, making it safer to take that shot -- or if he'd had a little more well-trained confidence in his own skills. (I wasn't there, and am going solely by the interview of him that I saw on YouTube, and maybe my suggested fixes would not have worked for some reason I can't picture right now; but it's most probable that Meli didn't think to change the angle because nobody had ever taught him how to do so. Few people think creatively under that kind of stress, and I take nothing away from his courage in trying to intervene or from his wisdom in holding fire.)
Another example: in multiple interviews, the story changed several times, but I think it's safe to say that Dan McKown would almost certainly have done better during the Tacoma Mall shooting if he had the training that allowed him to know that his actions would be legal, and enough confidence in his shooting skills to use his firearm to solve that deadly situation. Again, he's a hero and I take nothing away from that. I simply say that this brave man would almost certainly have done better if he had training than he did without it.
Anyway, my point is that while most incidents can be resolved without a gun, and most incidents that require a gun can be resolved without shooting, and most incidents that require shooting don't require a very high level of skill -- but incidents that involve multiple intended victims in a crowded public area and an active killer on the loose are different. They are incredibly rare, but they do require a higher level of skill and confidence to resolve successfully. That's why, if you're the type of person who absolutely would act to intervene in a situation like this, you should seek training to give yourself that level of skill and well-placed confidence.
My personal website: Cornered Cat