I love how people like to pretend the laws of war and theft don't apply just because it's the 21st century.
1) Dupe
Removing The Massachusetts State Flag
2) "Laws of War" are fluid and functionally applicable only when both sides are equally balanced, more or less. Laws of war in the currently-understood sense are also a relatively modern concept originating from the late 1800s. Nuremberg was the first "war crimes" trial and arguably Nuremberg was a show trial. The victors in Nuremberg wrote the rulebook. Not saying the Nazis and Japanese war criminals shouldn't have hanged, its just important to see all sides of the argument.
There's basically a few phases of colonial settlement in Southern New England. First phase was tense but more or less peaceful coexistence between the English and natives from 1620-1636. Then there was the Pequot War when Mass and proto-CT forces teamed up with the Narragansett and Mohegan to effectuate genocide against the Pequot. Between the Pequot War and King Philip's War in 1675 was a long period of peace where the English generally bargained and paid for land. RI's four founding cities were settled in this peaceful period between the Pequot War and King Philip's War. Roger Williams bought what is now Providence County, Anne Hutchinson's group bought Aquidneck Island, and Samuel Gorton bought more or less what is Kent County. Some deals were coerced, some were fair. RI tended to make fair deals with natives with the stronger Mass Bay and CT colonies being more demanding. Look into the history of Deerfield and Springfield and the English basically demanded food and support from the natives at first (although Springfield became a key trading point between natives and English). In RI, the colony was dependent on a peaceful environment between itself, natives, Mass Bay, and Plymouth.
King Philip's War was a response by the natives against English settlement and demands for land. Both sides exacted equal cruelty on each other. Tell a bunch of Narragansett and Wampanoag about medieval-era notions of chivalry or complex legal theories developed by Grotius or Coke, I'm sure they would listen back in 1675. Lots of towns got burned, many POWs taken, many brutal killings. English won though.
After King Philip's War, eastern Mass, RI, and most of CT were safe from native raids supported by the French. Northern and western Mass, NH, and Maine were the frontiers and as such lawless, violent, and dependent on mother colonies like Mass Bay. This period stabilized once Mass started asserting control over the NH/VT/Maine area in the 1740s and wars with natives in New England largely ended with the English victory in the French and Indian War.
Nations agree to laws of war in situations where the sides are balanced and there's a goal of trying to minimize collateral damage in the war. When there's a war between France and the Holy Roman Empire, or France and England, or Sweden and Denmark, both sides cannot destroy each other. But when the sides are fighting for survival, such notions of chivalry or lawful war go out the window. From the invaded's point of view, your goal is to get your pound of flesh and to drive out the invader. The invader's goal is to assert control over the area and effect pacification.
Self-defense doesn't just apply when there's a burglar in your home at midnight or when someone's trying to rob your wife. Nations, especially young nations in the early throws of their birth, have the right to self-defense and existence. Again, I'm quite sure an attempt to explain jurisprudence authored by 17th and 18th century legal scholars would've been either laughed at or ignored by the natives back then.