Robb was fighting against people who clearly had no real claim to the throne, who had conspired together to murder his father - a good man trying to uncover the truth. All while knowing those miscreants had crippled his brother for no good reason other than shame.
Robb Stark was totally in the right and his death was tragic.
Haha, what? Killing thousands for a "rightful claim to the throne" is just, but killing one to save the world is not? Give me a break!