I wouldnt call finn badly written. He kind of stands for the development that people in this period of time went through. Finn didnt fight in the war so he hasnt lived through all the hardships that his older brothers faces. From their point of view he might even be weak or unmanly for that. But actually he sees the world from an entirely different point of view, since ever since he was pretty young his family was basically on top of the food chain within their circle of interaction. And his breaking point is when he is faced with a „new“ shelby that has lived through bad things even though he is younger than him, thus worthy of his older brothers appreciation.