I agree,I slipped let my guard down in that situation.I am usually the opposite.I think you did GREAT!!
IMHO; the more important thing to be concerned with is improving you situational "awareness".
Ask yourself questions like what we may have done to prevent being robbed in the 1st place. (Keep the garage door closed and locked etc; maybe next time take turns moving around so at least one person can see any potential danger approaching.)
Do an "honest" self-evaluation to discovey "WHY" these BG's chose "you" to be their victim.
Remember the #1 rule of SD is "AVOIDANCE"
(Do things that will help you "AVOID" becoming a victim.)
Secondly; Be as "AWARE" as you possibly can to your surroundings.
Good questions to ask yourself throughout your day to help maintain "Awareness" are-
1. Where and attack may come from?
2. How to be more "prepared" should the attack come?
3. How to guard against an attack should it come?
Finally; Do a pro-vs-con "ASSESSMENT" of your activities and your situational "AWARENESS" throughout your day.
(This will help you "TRAIN your BRAIN" to better AVOID and remain ALERT to "possible" dangers.)
Now that there is something to think about having!.I agree, but the problem is, what happens when the state prosecutor does not think you are justified. All law is subject to interpretation, and his is the only interpretation that really matters (in regards to what charges are brought against you). The guy in your story, was obviously very lucky. Prosecutor's pick and choose who he prosecutes and if he thinks he can win and that being tough on anyone with a gun will improve his future political career, you could very easily end up being persecuted/prosecuted.
If the OP had one of these in his wallet pocket, he could have drawn without any suspicion. ....Yes Mr Criminal, I will give you all the money in my wallet.......BANG.
I think you have a good grasp on things.