Why was McCandless angry at his parents past?

Chris took a road trip after his graduation, during which, he goes to the neighborhood where he had spent his earliest years. Here, he gets enough information to piece together that his father had continued his relationship with his first wife in secret long after falling in love with Billie and fathering Chris, even fathering another son with her.

Chris doesn’t tell anyone about this discovery, but instead broods about it, letting the negative feelings build up more and more over time. Chris’s very strict moral code means that he is very unforgiving of his parents’ faults and failings, even though he often forgives his artistic heroes or close friends of equally or more profoundly immoral behaviors. As Chris’s resentment towards his parents grows, so too does it towards the rest of the world, as he starts to, for example, complain constantly about all the rich kids at Emory.