Scary situation. Honestly though, legally wise if he had ended up pulling the trigger on the guy. It probably wouldn't have looked to well in his favor.
Not that I think that is okay, just the way the world seems to be. Protecting one persons life, yours comes under threat. I see where the situation could be avoided, but we all know that isn't right. To just watch that, and shrug it off like it never happened.
You are allowed to prevent a forcible felony.
Getting a license plate number and following it is not initiating a crime.
If you rob a house, and shoot someone to protect yourself, you initiated the crime.
In this case, the BG committed assault or battery (whatever). He then pointed a firearm at you.
If the BG is close enough, duck down and use a 4000 pound projectile.