John Adams later said that the revolution began in the “hearts and minds of the people” around 1760, and I’ll take his word for it. The Boston Massacre took place in 1770, ten years into the period in which people in the colonies began to feel that they were being ill-treated by the royal government in London. They figured they had their own provincial legislatures and were quite capable of governing themselves, thank you very much.
Unfortunately, the king and his ministers saw the colonies as a source of revenue and attempted to squeeze money out of them without consulting the colonial assemblies. The colonists thought of themselves as British North Americans entitled to all the rights of Englishmen. The law in Britain going all the way back to Magna Carta said that the Crown could not impose taxes without the consent of the community being taxed. (“No 'scutage' or 'aid' may be levied in our kingdom without its general consent,” King John agreed in Magna Carta.) Inhabitants of the thirteen colonies were not asked for their consent, so they thought the taxes being imposed were illegitimate.
The taxes fell especially heavily on commercial activity, of which Boston was a prominent hub. The people of Boston felt they were being picked on by the British royal government. Matters got worse in 1768 when 4,000 British troops arrived and occupied the city, which had only 20,000 inhabitants.
So Boston was more or less a powder keg by 1770, when a group of toughs and unemployed men, some sailors and other fellows (condescendingly described by Adams as “a motley rabble of saucy boys, negroes and molattoes, Irish teagues and outlandish jack tarrs”) took it upon themselves to tangle with the soldiers guarding the Customs House. In the resulting mayhem, five colonists were killed or mortally wounded.
The incident crystallized the resentment Bostonians felt towards the British government. It did not itself spark the Revolution (the fighting at Lexington and Concord lay five years and one month in the future), but added to the grievances felt by the colonists.
The British officer in charge, Captain Preston, and eight of his men stood trial for murder. The captain and six soldiers were acquitted, and two soldiers were convicted of the lesser charge of manslaughter and were given relatively light punishments. These outcomes were viewed as unjust by the colonists and helped give rise to a boycott of British goods, which further inflamed the situation and enraged the British government.
The king’s ministers did not have the sense or good judgment to deal with the situation in American productively. Their intransigent attitude and reliance on coercion eventually led to the revolution and the loss of an extremely valuable part of the British empire.