Do I take him out because my instructor said it's cool and then end up getting myself in a heap of shat because the thug didn't even acknowledge me or point his weapon anywhere near my direction, so may life wasn't really in danger?
The way I understand the law just the fact that he was wielding a gun in a public place puts your life at risk. Legally. But then again, I'm no attorney.
I would probably make myself thin, draw my gun, and make sure that downrange is clear in case he does start shooting. If he doesn't shoot then he gets away with $35 and a Snickers, the clerk gets the scare of his life, and no one dies (not that I'm particularly sad when a bad guy gets killed "on the job").
But that's just me.