When I hear the word "massacre," I think of many people being killed in an incident that is unprovoked and unjustifiable. This is not the way it was at the Boston Massacre, yet we use that name for it. This is an example of perception and bias with regard to a historical event.
The massacre was not unprovoked. There was a lot of animosty between soldiers and Bostonians. Just before the massacre, people were throwing things (including rocks) at the British guards. The guards may well have feared for their lives. The massacre itself killed five people, which is a lot, but which does not reach the level of "massacre" by definition.
Perception and bias comes in because the British were the "bad guys" in our eyes. Because of that, we see their actions as uniformly bad and the actions of the Patriots (including the Tea Party and the violence against government officials) as good. We are biased in favor of our side and that strongly affects the way we look at historical events.