In his quest for revenge, Siu Yeun San murdered his son Siu Fung's adoptive parents (Kiu 3 Wai and his wife) and Shaolin teacher Yeun Fu. Kiu 3 Wai and his wife and Yeun Fu were good, kind, decent people who raised Kiu Fung to be a hero, and Siu Yeun San murdered them, causing his son terrible misery.
In light of this, would you have blamed Siu Fung if wanted to (or even in fact) kill Siu Yeun San? Biological father or not, Siu Yeun San committed reprehensible actions, and Siu Fung suffered terribly for them.