The main way that pathos is created in the audience is through the interviews that he conducts with people in McCandless's life.
In particular, two interview sessions produce the most sadness and empathy both for Chris and for those who loved and lost him.
The first one is with his family. His mother, father and sister are very obviously distraught about his death, so much that there is crying during the interview and expressions of guilt and helplessness.
The other particularly striking interview is with the man who lost his own family in a car accident, Ron Franz. This man changed his whole entire life in order to live the life that "Alex" encouraged him to lead.
The initial response to hearing McCandless's story is to think that he is insane or stupid for his decision. However, thru his use of pathos, Krakauer makes the readers understand "Alex" on a more personal level and makes his death seem tragic instead of ridiculous