The punishment should fit the crime. No one will support the death penalty for shoplifting because it's out of proportions to the crime. Robb betrayed the Freys and dishonored himself. Robb sucked at being king but Walder taking the punishment to that level wasn't justice. It's the same morality dilemma between Jon Snow and Janos Slynt. Slynt insulted Jon but in no way did it justify Jon killing him. Walder was wrong to murder Robb while Jon was wrong to murder Janos.