The depiction of that battle in the film is based on the aftermath of the real siege of Fort William Henry. The film greatly compresses and simplifies the encounter.

The French had allied themselves with various Native American groups. After Fort William Henry surrendered, the French tried to make sure that the tribes were clear that the British were not to be harassed once they left. Whether the Native American's didn't care or didn't understand isn't quite clear. Varying groups and tribes attacked the British column almost from the moment the battle was over, plundering and killing several hundred and capturing more over the course of a few days. These tactics were mainly effective due to the fact that the British had no gunpowder to protect themselves after the surrender, and the French only provided minimal and reluctant protection.

There wasn't a single massacre like in the film, more like a long running harassment as the British retreated. In fact to call it a massacre at all probably isn't accurate. The French general later secured the release of many of the British captives, though he never quite disapproved of their actions.

Source: The First Global War: Britain, France, and the Fate of North America, 1756–1775

http://www.historymuseum.ca/cmc/exhibitions/aborig/haida/havwa01e.shtml

The Haida of the northwest coast had taken down a few war ships. What is not mentioned here is their slave trade (not of Europeans or colonists necessarily)