Mass. Gen. Laws ch. 140 § 156
Any person may kill a dog which suddenly assaults him while he is peaceably standing, walking or riding outside the enclosure of its owner or keeper; and any person may kill a dog found out of the enclosure of its owner or keeper and not under his immediate care in the act of worrying, wounding or killing persons, live stock or fowls, and if any person shall kill or attempt to kill a dog so found, and in the act of worrying, wounding or killing persons, live stock or fowls, he shall not be held liable for cruelty to the dog unless it shall be shown that he intended to be cruel to the dog, or that he acted with a wanton and reckless disregard for the suffering of the dog. A person killing or wounding a dog under the conditions set out in this section shall promptly report to the owner, animal control officer or police officer such killing or wounding. Prompt killing of a wounded dog, or a prompt report to the owner or to a dog officer of the wounding of the dog, shall be considered evidence of sufficient regard for the suffering of the dog.
Mass. Gen. Laws ch. 140, § 156