The Boston Massacre in 1770 represented a further breakdown in relations between the British and the American colonists. Moreover, it fanned the flames of revolutionary fervor already spreading widely and rapidly throughout America.
The basic facts of the Massacre are not in serious dispute. On March 5th, 1770, a self-proclaimed patriotic mob clashed with British soldiers. The mob threw stones, sticks, snowballs, and anything else they could lay their hands on at the troops. In response, the British soldiers opened fire, killing several of the protesters. Irrespective of the rights and wrongs of what happened that day, the consequences were grave. Revolutionary pamphleteers and agitators made huge political capital out of the Massacre, using it as an example of the British tyranny that the Americans needed to overthrow. To many, it now seemed that some kind of armed conflict was inevitable.
Tensions had been building between British troops and the citizens of Boston for some time. Citizens came to resent the presence of British soldiers, seeing them as an occupying force and an instrument of tyranny. These tensions came to a head on the day of the Massacre, which began when a mob of fifty Americans attacked a soldier on guard duty. In that sense, one could say that it was these citizens, and those who participated in the ensuing disturbance, who were ultimately responsible for what happened later.
Yet at the same time one could also argue that the response of the British troops was excessive and unwarranted. They were not being shot at by the protesters, and so firing directly into the rioting crowd could be seen as disproportionate. Captain Preston was aware of the gravity of the rapidly deteriorating situation, and it's notable that he didn't personally give the orders to shoot, knowing as he did that it would be likely to inflame the situation further. But the soldiers did shoot all the same. In all likelihood, the soldiers panicked, not least because they hadn't been properly trained in dealing with civil disturbance.
At the subsequent trial, Captain Preston and his men were acquitted of murder. Famously, they were represented in court by John Adams, future president of the United States. Perhaps the fairest way to answer this question is to say that, although Preston and his men were not legally responsible for the Boston Massacre, there seems little doubt that they bore some moral responsibility for the bloodshed.