Florida Stand your ground allows the use of deadly force to protect yourself without the requirement to flee, as long as you have the legal right to be there and you are not the aggressor.
You do not have to be in your home, just in a place you have a legal right to be, which is why it provides protection from prosecution in or out of your home.
Massachusetts has a "castle doctrine" in theory, thanks to former Governor Ed King, but as with every law in MA that is supposed to allow you to use a weapon to defend yourself, you always run the risk of getting jammed up and bankrupted by legal fees trying to defend yourself from the Commonwealth.
The guy referenced above better hope his defense team can get a Doctor on the stand that will convince a jury that the perp was already dead when the second shot was fired, or that the first shot was going to be fatal (death before medical help arrived, death from the first shot was going to be fatal even with medical intervention) and the second shot may have caused immediate death, but death was imminent anyway.
A person is allowed to act in self-defense. If evidence of self-defense
is present, the Commonwealth must prove beyond a reasonable doubt that
the defendant did not act in self-defense. In other words, if you have a
reasonable doubt whether or not the defendant acted in self-defense, your
verdict must be not guilty.