J.L. Bell at Boston 1775 tries to answer this question. Here is a taste:
Bostonians started to call the killings on King Street on 5 Mar 1770 a “massacre” almost immediately, according to the official record. The minutes of the emergency town meeting that started the next day begin:
At a Meeting of the Freeholders and other Inhabitants of the Town of Boston at Faneuil Hall on Tuesday the 6th. Day of March 1770 – 11 O’Clock A:M; occasioned by the Massacre made in King Street, by the Soldiery the preceeding Night . . .
Upon a Motion made it was Voted, that if any of the Inhabitants present could give information respecting the Massacre of the last Night, that they be desired to do it in Meeting, that the same might be minuted by the Town Clerk
That clerk was William Cooper, and it appears he was the person who started to apply the term “massacre” as he took notes at that meeting.
By the end of that town meeting that afternoon, Cooper was writing the phrase “horrid Massacre.” On the afternoon of the 12th, that had become “the late horred Massacre.” The latter meeting had chosen a small committee headed by James Bowdoin to write Boston’s official report on the event, which had the title A Short Narrative of the Horrid Massacre in Boston.
In choosing that word, Boston Whigs strengthened the links they perceived between them and government reformers in London. The term echoed the Massacre of St. George’s Fields, which had taken place in London in May 1768. A crowd had turned out to show support for the radical politician John Wilkes. Magistrates “read the Riot Act,” ordering the people to disperse. When they didn’t, soldiers fired at the crowd, killing six to eleven people.
Read the rest here.