When a government uses its army to start shooting its citizens in the street, this is usually a sign that things are not going very well between the government and the people. Now that's not exactly what happened in the Boston Massacre, because the soldiers were not ordered to fire on civilian citizens by their officers, but opened fire on their own. Still, for the army to start killing the people is usually a sign that the relationship between government and the governed is not in great shape.

Of course, when the army starts killing the citizens, this does not always lead to revolution or civil war, but it often does.

The fact that the army was in Boston in the first place was a sign that all was not well between the government and the citizens of Boston. Sending in the Army to enforce laws is not generally necessary.

Four regiments of the British Army had been sent to Boston in 1768 to maintain order after various riots in Boston protesting against the Townsend Acts (which had imposed new taxes without consultation or representation).

The Boston Massacre itself grew out of an incident in 1770 where the townspeople of Boston were taunting a British sentry. The sentries' friends came to his aid. Rocks and other things were thrown. Then the soldiers (without orders from officers) fired on the crowd, killing 5 Bostonians and injuring 6 others.

Even though the soldiers' firing on the crowd was not a deliberate act by the Army, the already poor relations and hostile mood between the Army and the citizens, understandably, deteriorated farther.

The people of Boston also thought that although the shooting was not ordered by the British government or their officers, the occupation of Boston by the army was the fault of the government, and the failure of the government to keep their army under control and prevent them from shooting people was also the fault of the government.

So, not surprisingly, feelings of hostility and suspicion towards the British government were amplified by the Boston Massacre.

In all, the Boston Massacre was one of a chain of incidents which led to a breakdown in trust and a breakdown in loyalty between the citizens of Boston and the British Government.

Eventually (5 years after the Boston Massacre) this steadily deteriorating situation would descend into war.

It is because the Boston Massacre was a key incident in, and contributor to, the ongoing deterioration in trust and loyalty between the colonists and the British Government that people see it as one of the incidents which contributed to the causes of the American Revolution.