The Boston Massacre was a historical event where the British soldiers shot an open fire at the American colonists. They shot the people because of their fearfulness which later provoked the colonists to fight back even more. 5 people died from these shootings. It was a brief but bloody fight.
Adding on, the definition of massacre is a violent murder a large group of innocent people. The soldiers killed 5 colonists which didn't seem much but it still consisted of murdering blameless people.