Since he died "off screen/stage" in the books, there's a fan theory that he isn't actually dead. IIRC the only way we know he is dead is from a conversation Brienne had with the priest/maester (can't recall exactly) who tended to him as he died and buried him. The priest/maester then left his hound helmet on top of the grave, and it was stolen. Then rumors of the "hound" (someone wearing the hound's helmet) raping/pillaging are brought to kings landing. Obviously, this is due to whoever stole the helmet. However, the theory is that the priest/maester was actually talking about the death of the "hound" personality in sandor, and he is really still alive and acting as his own man now. when we first meet the hound he is best known for being the younger brother to gregor/the mountain, and unquestioningly loyal to the crown (robert/joffrey). so the hope is he shows up again later as a changed man, thus completing his character arc from thug/enforcer to true knight (which is ironic since he hates knights).