The reason you are getting different answers is that the issue is not simple. You can't just point at one thing and say that's the reason why.
It turned vicious because both sides plundered the land of the other.
It turned vicious because people were already divided in their loyalties and a divided people, if they are fighting to win, will resort to anything to make winning an outcome.
It turned vicious because when the righting actually started guerrilla tactics were employed by both sides really although they better known side was definitely the patriots. Guerrilla warfare is always vicious, but sometimes it is the only tool you have.
It turned vicious because Britain did not police the situation. They thought they could but it turned out that they could not. The divisions were just too intense and too much like war to do anything, or so they thought.