Massachusetts Castle Doctrine

MGL Ch. 231 Sec. 85U protects a person from civil liability 
provided they meet the standards of MGL Ch. 278 Sec. 8A

MGL Chapter 278

Section 8A. In the prosecution of a person who is an occupant of a dwelling charged with killing or injuring one who was unlawfully in said dwelling, it shall be a defense that the occupant was in his dwelling at the time of the offense and that he acted in the reasonable belief that the person unlawfully in said dwelling was about to inflict great bodily injury or death upon said occupant or upon another person lawfully in said dwelling, and that said occupant used reasonable means to defend himself or such other person lawfully in said dwelling. There shall be no duty on said occupant to retreat from such person unlawfully ​in said dwelling.

MGL Chapter 231

Section 85U. No person who is a lawful occupant of a dwelling shall be liable in an action for damages for death or injuries to an unlawful occupant of said dwelling resulting from the acts of said lawful occupant; provided, however, that said lawful occupant was in the dwelling at the time of the occurrence and that he acted in the reasonable belief that the person unlawfully in said lawful dwelling was about to inflict great bodily injury or death upon said occupant or upon another person lawfully in said dwelling, and that said lawful occupant used reasonable means to defend himself or such other person lawfully in said dwelling. There shall not be a duty on said occupant to retreat from such person unlawfully in said dwelling.