While the Patriot is notorious for being littered with historical inaccuracies and the portrayal of historical myth as truth, war crimes were something that occured during the Revolutionary War - but unlike their portrayal in that film, these war crimes were perpetuated by both Patriots and Loyalists.

The key to understanding why these war crimes occured lies in understanding two vital aspects about the Revolutionary War that, unfortunately, has been ignored by many British and American scholars:

The first being that, at the time of the conflict, there was no real separation between American and British. To those living at the time of the conflict, the Revolutionary War was not a conflict between two peoples, rather, it was a civil war between the British themselves. In many ways the Revolutionary War was America's first civil war as the vast majority of those who actually fought in the conflict were American-born, only a minority of those who fought during the conflict were British-born; and this minority was primarily concentrated in the officer class. It wouldn't be until the later stages of the conflict that British-born soldiers would make up a majority of the Loyalist forces, this only having occured once regular reinforcments made it to North America from other parts of the Empire.

The second of these vital aspects being that the conflict was a massively unpopular affair. At modern estimates, about a third of Americans supported the Patriot cause while a fifth supported the Loyalist cause - the rest of Americans, a majority, saw no real reason to revolt against British rule or to actively support British rule. By the time the war came to an end most of these American Loyalists left the newly founded United States, with roughly 100,000 emigrating to Canada. Even William Franklin, the eldest son of Benjamin Franklin, emigrated to Britain where he lived till his death in 1813.

With this in mind, to the majority of those living in America at the time of the Revolutionary War, the conflict was something to have nothing to do with. In their eyes the Patriots were not revolutionary heroes, rather they were rebels fighting for a cause they had little concern for; while, in the same vein, they saw the Loyalists as die-hard monarchists who were simply fighting to maintain the status quo. When it comes to the Patriots or Loyalists, they were fighting against traitors who had to be defeated at all costs.

As a result, the American Revolution was a conflict abundant in war crimes and violence committed against the citizenry and prisoners of war alike. Men, women, and children were murdered, raped, and abused while their property was looted and destroyed. For the Patriot forces, this was done to weed out traitors to the Revolutionary cause, while for the Loyalists this was done for the game goal - to weed out traitors in revolt against King and Country. These war crimes were conducted not only as a result of political fervour, but perhaps more importantly, as a result of a need for resources and supplies - as both sides largely shared the same resource pool and supply lines. Though both the Continental Congress and the British Parliament attempted to put an end to this violence, due to the fervent idealism of both Patriot and Loyalist forces their attempts failed to achieve any real results.

I hope this answer is helpful.

Sources:

Benjamin Franklin, Patriot, and William Franklin, Loyalist by Sheila Skemp (1998)

Empire: How Britain Made the Modern World by Niell Ferguson (2004)

Scars of Independence: America's Violent Birth by Holger Hoock (2017)