Wolf murdering David's parents. It's an even bigger tearjerker after you read the Word of God.
The entire fanfiction becomes incredibly more sadder after you read the Word of God. David was alone this entire time and Fox was nothing but a hallucination created by his own disturbed mind to keep him from going absolutely insane...
David himself is a pretty tearjerkish character. He is abused by his parents, has no friends (specially after the incident) and has schizophrenia, which means that Fox was never there. He never really had a true friend by his side...
