Were the sons of liberty justified in their action?

As to whether the Sons of Liberty were justified in their actions, I must leave that to you to decide. Since the other educator has already addressed the Boston Tea Party, I will address other aspects of the organization's activities.

First, the group was considered a paramilitary organization. Its members were made up of patriots who objected to "taxation without representation." The Sons of Liberty resented the British government for levying varied taxes on the colonies while denying colonists a voice in the English Parliament. The Sons of Liberty originated in Boston, with later chapters forming in New York and the rest of the thirteen colonies. The organization was initially formed to protest the Stamp Act. 

One of the organization's first acts was to terrorize Andrew Oliver, a public official with the authority to enforce the Stamp Act in Massachusetts. Accordingly, the Sons of Liberty created an effigy of Oliver, which they paraded through Boston's streets and later burned. Angry citizens swelled the ranks, and the mob marched to Oliver's home, where they smashed windows and made off with expensive furnishings. Oliver eventually resigned his commission.

Immediately after, the Sons of Liberty demanded that Thomas Hutchinson, the Loyalist Chief Justice of Massachusetts, disavow the Stamp Act. When Hutchinson refused, the Sons of Liberty caused extensive damage to his home and fought against his supporters. Certainly, the Sons of Liberty were feared, and it was said that the group pioneered the horrific "tar and feathers" punishment. However, the cruel torture actually originated from the days of Richard the Lionheart. 

Although the Sons of Liberty did not subject any New England stamp commissioners to the dreaded punishment, they did torture other loyalists in such a manner. First, the victim was stripped, and then molten tar was poured on him. Thereafter, feathers were thrown at the sufferer so that they stuck on his tarred body. Because the Sons of Liberty were so feared, every stamp commissioner resigned his commission in the thirteen colonies, leaving no official to collect the stamp taxes. The Stamp Act was eventually repealed.

The Sons of Liberty later went on to mobilize groups that formed the backbone of the American Revolution. Later, in the 19th century, abolitionist Joshua Giddings established a committee called the Sons of Liberty. Its purpose was to consider the use of force against slave catchers. 

Were the Sons of Liberty justified in their actions? The answer will depend upon your perspective about violence.