Considering the operative Confucian ethic of 'righteousness' 義 guiding Lord Guan Yu's decisions, how is Guan Yu able to rationalize his seemingly vain decision to spare the life of Cao Cao, given that this decisions appears to stand in direct contravention to Guan Yu's obligatory Confucian duty as ruler (君) to act only in the interest of his subjects (臣), lest he give up the 'Mandate of Heaven' (天命)? Given the subsequent collateral damage, was this decision consistent with the Confucian ethic of 'righteousness'? Why or why not?

Guan Yu, a great warrior renowned for his bravery, had temporarily committed to serve Cao Cao. His decision was largely based on protecting the family of Liu Pei, his “sworn” brother, to whom he owes a more lasting bond of service. Although Cao Cao is his inferior in social status and, generally, in behavior, they share a belief in the importance of generosity. When Guan Yu sees it is necessary to repay Cao Cao’s generosity, he does so by slaying Cao Cao’s rival. Once this repayment is made, he decides to return to Liu Pei. Cao Cao stops his troops from following Guan Yu as they had wanted to do.
Later, however, the two men face each other in battle at the Red Cliff, and Guan Yu defeats Cao Cao. In tribute to this ethos of generosity and recalling that Cao Cao had earlier allowed him to depart, he spares the vanquished man’s life. Although some of the repercussions did threaten his people’s interests, his decision was based in his own sense of righteous conduct and thereby served as model behavior for them, thus in the long run serving their interests and not violating his obligation.