Some guys from another business across the street heard the shooting and saw the guy leave. They jumped in a car, got next to the man with the shotgun slung on his back, and shot him to death from their car.
This led to a lot of education on this point of the law for us. The DA said that since the guy had just used the weapon to inflict bodily harm, and still had it, he presented an immediate danger to the public even though his weapon was slung and he was riding away on a bike.
Who knows who his next victim might be? Nobody is obligated to find out. He still had the weapon, and the 2 civilians made a "good shoot" and got kudos all around, from the cops to the mayor. (They were worried at first and came up with a cock and bull "accidental discharge/warning shot" story first to cover themselves. No need. They were not even booked.)
I think the situation in Mass is the same. The perp had just used the gun, and still had it. He's fair game, because the presumption is that he could use it again at any moment to kill the next person he saw.
LOL Travis, this happened in my town...I remember it well. They not only shot the bozo....they ran him over too! The tenth anniversary of that incident just passed and they tried to re-interview the heroes, but one had already passed away and the other said he knew he'd done the right thing, but that he was "haunted." I guess good people handle things in different ways. I think I not only would have run him over, I'd have locked the wheels on him.