The issue isn't how long one has lived in Massachusetts, but rather whether one intends to continue living in Massachusetts, or one of the other 49 states, all of which have pretty much the same rule on this point. Neither is the issue whether one agrees or disagrees with the rule or feel that it denies one the opportunity to achieve true "justice."
The reality, rather, is that in this state (and virtually all others), the privilege of using deadly force is limited to the necessity of self-defense and the necessity of self-defense ends when the bad guy turns tail and runs. The next shot you fire is not for defense but retribution, and retribution will land you in jail. Fact of life, and just about as important to learn as tap-rack-bang.