In Into the Wild, what did Chris learn about his family on his first trip across the country and how did this knowledge affect him?

During his first trip across the country, Chris visited the Southern California neighborhood where he grew up and met up with some old family friends.  From these friends he learned the details of his father Walt's messy divorce from his first wife Marcia.  Walt had continued his relationship with Marcia long after he began his relationship with Billie, Chris's mother.  Walt had led a secret double life for a time, going back and forth between two households, and had fathered a child with Marcia two years after Chris was born.

Although he never let on to his parents that he knew the truth, Chris brooded over what he considered to be his father's duplicity and betrayal for years.  He told his sister Carine that this knowledge made "his entire childhood seem like a fiction".  There is little doubt that Chris's discovery about his father's secret life festered inside him, fueling the otherwise seemingly inexplicable rage and resentment he felt toward his family, and leading to his finally shutting them out completely from his life (Chapter 12).