One reason why the British soldiers fired on the Boston colonists during the Boston Massacre was because their were objects being thrown at them. These soldiers were harassed by an angry mob and hit by objects while protecting the Customs Office in Boston.
Another reason why the British soldiers fired on the colonists were because they were afraid of what might happen. Colonists, at this time, rebelled constantly against any symbol of the British government. This included tax collectors, who were tarred and feathered by colonists. These soldiers could have been worried that the colonists might attack them physically.