The Boston Massacre really did not have a purpose.  It was not something that the British set out to do.  Instead, it happened spontaneously.  To the extent that it had a purpose, its purpose was to prevent a group of British soldiers from being harmed by a crowd of angry colonists.

In the spring of 1770, British soldiers had been posted in Boston for almost two years.  They were there to enforce tax and other laws that the colonists disliked.  The colonists resented the fact that the soldiers were in their city.  This led to various incidents between soldiers and colonists.

On March 5th of that year, a lone British sentry was guarding a customs house when he started to be harassed by a crowd.  Eight more soldiers came to help him, and the crowd began to threaten them, taunt them, and even throw things at them.  The soldiers were under orders not to fire, but one of them did end up firing, presumably because he was hit by a thrown object.  This caused other soldiers to fire as well, killing three colonists immediately and wounding others, two of whom later died.

From this account, we can see that there was no real purpose behind the massacre.  The soldiers had not been sent out to kill colonists to make a point or anything like that.  Instead, the massacre was an accident that came about because of tense relations between American colonists and British soldiers.  The only purpose behind it was the soldiers’ desire to keep themselves safe from what they perceived as threats.