The only reason police are allowed to fire if they believe they see a weapon is because of the nature of their jobs. Cops do run into bad guys with guns all the time. People really are trying to kill them, very often.
It's not comparable to average citizens. I do find it interesting how often some gun enthusiasts feel like they are allowed to do exactly what cops do, though. They don't see that they have not been screened or trained or hired to be our cops!
I have guns for protection, and I would certainly protect my neighbor's life, too, but I'm well aware that I'm no cop and that life-and-death situations take a ton of training to handle. I'd only pull out my weapon if things were crystal, crystal clear.
Before that, I'd RUN

And I have run from violence before, really. Things go down so fast that decision making is really hard. Most people are terrified of being wrong if they are the ones in possession of a lethal weapon. Unfortunately, some people think they can do too much too fast and call it fair. The SYG law seems to encourage that, or people's understanding of it does, anyway.