DaoistWDfd8h
There is no fostering I see. Fostering start usually from a young age, 6 or 7 or 8, so there is no relationship with the Starks or reason to go to war. But the mad king could just as well murder Steffon, because there is a difference between a hero on a path of vengeance for his comrades, and a Lord who seized an opportunity to betray and seize a King's position.