I am taking the classical stance.
He is a king which means that he’s the closest to god than any other human being around him. If we are taking into consideration the divine being belief system that everyone was taking on at the time. he is above everyone. therefore any sin that he would have made against anyone is justified because he is above them. So any sin against him is much more serious. Therefore his saying ” I am a man more sinned against then sinning” I agree with.