These "what if" stories and "I'd do this" sound really good and might make you feel a little better about yourself, but they'll never happen that way.
There is a risk your taking when you go out of your way to help others. Not just the action movie risk, but the legal risks after the action. If you aren't willing to take those risks then you should just stay in the basement apartment with your mom.
I'm bothered by this because, I don't want to be paying a lawyer to explain why I felt it necessary to kill a guy who kicked my door in carrying a sword.
If somebody was banging and kicking at your front door (for what ever reason) would you answer it?
By not enforcing the law it's saying that it doesn't matter what others do to you, as long as they did it with good intentions.