sometimes you gotta wonder with this crowd. They're so afraid of MA that they'd never use their gun yet for some reason carry anyway. I think the gun might be like fashion accessory to them or something that just makes them feel cool.
Assuming no crazy extenuating circumstances in the OP, it's a clean shot on the robber. You'd probably be arrested, detained, and then released. People think in MA you're ALWAYS going to get charged if you ever use your gun in self defense or defense of another and it's just plain untrue. Hell, if I really believed that I'd never have gotten a license to carry.
What happens in 999/1000 such robberies? The robber gets away with a couple hundred bucks.
Some years back, a woman in Arlington was in her home when she heard noises on her enclosed front porch. She opened her front door to find an intruder on the porch. He walked towards her, through her open front door. She drew her gun (she was properly licensed) and ordered him to leave. He laughed and advanced on her. She shot once, hitting him in the neck. He fled and was apprehended shortly thereafter.
Her LTC was suspended and her guns and ammo confiscated. After about six months, the DA decided it was a good shoot. At that point, the chief was going to permanently revoke her LTC. Her attorney was able to convince him otherwise. This was a black-and-white good shoot in her own home. Even though she was never charged, she was still dragged through a legal wringer that cost her thousands of dollars and untold emotional trauma.
In 2009, a psychiatric patient attacked Dr. Astrid Desrosiers. He was on top of her, stabbing her, when an LTC holder who heard the screams ran into the room. He ordered the attacker to drop the knife, and when the attacker didn't comply, he fired, killing the attacker. This was another completely clean shoot. The man was hailed as heroic in the Boston Globe. And yet it took the DA 6 months to clear the case and during that time he was unable to work.
If you think a situation like this would be cleared up quickly in MA, you are very mistaken. It will take months and cost thousands of dollars, provided it is an open-and-shut case -- if not, it will be much, much worse.
Back when I took LFI-1, Ayoob said that surviving a gunfight is the second worst thing in the world that can happen to you, and he's spoken to many such survivors.
If I use my gun to protect the clerk, he isn't going to pay my legal fees. If I'm an emotional wreck as a result of the press coverage and potential jail time (like the Zimmerman fiasco), the clerk isn't going to pay my salary. If I get shot in the process, the clerk isn't going to pay my medical bills. If I lose my job because my employer gets freaked out that I was carrying a gun, the clerk won't find me a job.
As I said above, I carry a gun to protect me and mine. I'm not a cop and intervening in third-party situations is a recipe for disaster. I'm not saying that I wouldn't ever intervene, but intervening would certainly not be my default choice.