There is no duty to retreat inside your home against strangers.
Some states even have that extended to the property line.
Some states even have that extended to the property line.
I don't know Massachusetts law, so I don't know if there was a duty to retreat. But he also didn't have to fire through a closed door. That's just plain dumb, regardless of the law. I'm no liberal, but I do understand some basic concepts of self defense and shooting a firearm.