I don't have the actual MA law code available from a quick search, but I have read it before.
Here are a couple of references I found:
http://www.massgunlaw.com/hot_issues_in_massachusetts_fire.htm
" If you are in your own home, there is no duty to retreat, but you may use deadly force only to repel an intruder's deadly force attack against you or your family. "
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http://www.findarticles.com/p/articles/mi_m3197/is_n8_v36/ai_11199334
This is from Ayoob: "The one exception to this was a period of time from the late '70s to the early '80s in Massachusetts, where it was widely accepted that the caselaw of Commonwealth of Massachusetts v. Lynn Schaefer held that when attacked in one's home, one had to try to climb out the windows before resorting to a gun in self defense. This was actually a rather nebulous case and never got to the higher courts where it would have been shot down. In any case, during a brief flash of collective sanity between the Dukakis administrations, the Bay State elected John King as governor and he promptly signed into law a "castle doctrine" that brought Massachusetts into line with the rest of the civilized world.
Massachusetts was neither the first nor the last state to enact what pro-gunners call a "castle law" and anti-gunners call a "Death Wish law" or a "kill thy neighbor" law. Be advised that these laws DO NOT permit the indiscriminate shooting of any intruder caught inside the dwelling! They permit the use of deadly force ONLY when the intruder seriously poses an immediate threat to the life and limb of someone therein. Castle laws have been widely and horribly misinterpreted by a large number of professionals in the firearms field who should have known better."
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Now it would be preferable, of course, if the law more clearly stated that anyone in your home uninvited poses a de facto threat, but the MA law is still better than some states.