Dr. Wu Ching Ngau, the "Medical Saint" of HSDS, was infamously known as a callous doctor who refused to treat any patient who was not affiliated with the Ming Cult. The reason he gave for this is that one man he healed, the leader of the Mt. Hua Sect, betrayed him and caused the death of Dr. Wu's younger sister. A second reason was that Dr. Wu had angered his wife (the "Poison Saint") by treating a patient whom she had poisoned.
Dr. Wu talked tough, but it seemed that deep down, he was a compassionate physician who couldn't help but fulfill his moral and professional duty. He always claimed he'd kill Cheung Mo Gei, but during the five years that Cheung stayed with him, Dr. Wu never seriously tried to harm him. In fact, other than being a little brusque, Dr. Wu treated Cheung Mo Gei quite well.
Most tellingly, when Gei Hiu Fu and several others injured by Granny Golden Flower came to Butterfly Valley for Dr. Wu's treatment, he was willing to instruct Cheung Mo Gei to treat all of them...including, interestingly enough, a member of the Mt. Hua Sect that Dr. Wu supposedly so hated.
It seems that Dr. Wu's bark was worse than his bite; ultimately, despite his infamous reputation, he was a good-hearted doctor whose natural inclination was to help the wounded and sick.