Fair question--especially given that the British were technically the people charged with preserving the peace at the time. Not to mention it seems that the British and colonists were closer than most people realize at that point in the move towards Independence. See this Boston Magazine article.
On the other hand, the use of deadly force was not a first option in self-defence at the time the founders were alive. That meant that the British soldiers would have had to have done all they could to de-escalate or remove themselves from the situation before deadly force would have remotely been acceptable. When I say that, I mean that deadly force would still be unlawful had there been lesser means to have stopped the threat.
I should add that some other nations require that the police and other security forces use the minimum force necessary to stop the threat. I talk about this in Defund the Police is Utopian, Misinformed, and Misguided.
Let's toss in there was a defence:
The statement issued by members of the Sons of Liberty, including Samuel Adams and John Hancock, painted the event as a malicious and unprovoked slaughter. They attest that the Massacre was retribution for a quarrel three nights prior between soldiers and colonists. Captain Preston, the British commander on duty on the night of March 5, was reported to have ordered his men to fire upon the colonists on King Street, "without the least warning."
William Taint, a Bostonian who witnessed but was not directly involved in the events of March, provided testimony during the trial of the British soldiers. He maintains that a group of colonists was gathered outside of the British Customs House when a formation of British soldiers took position outside of the building. Colonists were yelling, "Fire, fire, and be damned," and throwing snowballs at the British soldiers. Taint heard a musket discharge and then the word, "Fire" yelled by an unknown speaker, after which several more shots were fired.
Taint's account differs from that provided by Adams and Hancock in several respects. Firstly, Taint clearly states that the colonists were taunting and throwing snowballs at the soldiers, while Adams and Hancock portray the colonists peacefully going about their business. Taint also brings an element of uncertainty to the question of who yelled, "Fire." The former account clearly states that Captain Preston issued a direct order to fire, while the latter implies that it may just as well have been a colonist who shouted, "Fire."
Captain Preston's testimony during the trial offers a third source of information regarding March 5. He states that his men were protecting the Customs House from theft by the colonists when they were physically and verbally assaulted. While Taint saw only snowballs being hurled at the British, Preston reported that his men were also beaten with clubs. His soldiers responded by firing upon the colonists, later claiming that they heard the command to fire and assumed it came from Preston. Preston blames members of the mob for yelling, "Fire," and (unsurprisingly) denies issuing any such order.
The outcome was Captain Preston was found not guilty. The remaining soldiers claimed self-defense and were all found not guilty of murder. Two of them—Hugh Montgomery and Matthew Kilroy—were found guilty of manslaughter and were branded on the thumbs as first offenders per English law.
See also:
https://www.history.com/topics/american-revolution/boston-massacre
