Right on, Art! I don't think any of us would be so dumb as to start a firefight in a crowded restaurant. Let the BG take the money and flee.
Just get a good description of the BG so the police have something to work with; including (if it is safe to do so) a description of accomplices, vehicles, etc.
If he begins shooting people, a la Killeen, anyone has the right in Texas to use force or deadly force to prevent or stop the unlawful commission of deadly force. (OK, OK, don't start with the "what if the BG is a LEO?" arguments.)
But, "justified" does not mean "required" (to use force or deadly force).
It is usually best to stay out of the fray. What if the BG has unknown partners in the crowd? What if the off-duty 21 y/o deputy constable and/or a poorly trained, hypercop wannabe see you with a gun and think you are the BGs buddy?
The possibility of unwarranted death, injury, property damage, criminal charges, civil suits, etc. is just too great unless the BG starts shooting.
Frankly, even if BG shoots one person, the crowd is still there, your other problems still exist, etc. I probably would not open fire until my family, friends, or I were under the immediate threat of imminent serious bodily injury or death; to BG begins to assassinate people.
If he begins shooting a number of people, and I have a clear (safe) shot and those I am responsible for are safe, then I'll Mozambique him and BG will mean "Been Greased".
BTW, I'm only talking about non-LEOs. LEOs have their own rules, customs, policies, obligations, etc.
As y'all pointed out, if your use of force negligently injures an innocent party, immediately check both shoes -- you done stepped in it, big and bad.
For Texans, I recommend a careful reading of the Penal Code, especially chapter nine.