To assume any hypothetical to be true is generally folly, including the "good guy with a gun" scenario as having no effect or a negative outcome. It simply can't be known. 

But I'll violate my own admonition: I think it's more likely than not that a bad guy confronted by armed resistance would flee, although not likely to surrender. 

Again, there is no way to know this, just as there is no way to know that the good guys will definitely end up shooting innocent bystanders.

(And whether the Texas guy "returned fire" or not is a pointless distinction. He, by official accounts, ended a terrible situation. Good for him.)