Immediately after his family was murdered by Chinese wulin warriors at Ah-Mun Pass, Siu Yeun San vengefully killed many of those who had attacked his family. This is understandable. Nobody could blame him for that.
And yet, towards the end of the fight, Siu's hatred seemed to vanish, and he felt only grief (for his wife's death) and remorse (for the lives he had taken). He didn't kill all of the people who assaulted his family; he spared quite a few of them. Then, because he felt remorse for having violated his promise to his teacher to never kill a Han person, Siu committed suicide in repentance.
Those are the actions of a compassionate, good-hearted man who, under normal circumstances, wouldn't willingly kill anyone, and only did so because he had been driven to understandable momentary rage by an unprovoked act of savagery against his family.
For nearly thirty years, Siu Yeun San did not reappear to take revenge against the remaining participants in the Ah-Mun Pass Massacre. He seemed to have forgiven them, or at least, decided not to pursue further acts of vengeance against them.
Then, suddenly, not long after his son Siu Fung's heritage and origins are revealed, Siu suddenly reappears and goes on a murder spree...killing many of those whom he had spared years earlier.
Why this change of heart on Siu Yeun San's part? He went from seemingly having no interest in revenge for nearly thirty years to becoming bent on revenge against people he had previously spared.