Chris McCandless showed his compassion through his sense of black and white. McCandless saw the world as black and white, and he felt that people with money were vile, and those without it were to be shown compassion. He lacked the ability to show his parents compassion, because he was busy resenting were and what they were. But, in high school,
"on weekends...McCandless would wander the seedier quarters of Washington, chatting with prostitutes and homeless people, buying them meals, earnestly suggesting ways they might improve their lives" (p. 113)
His mother stated that Chris did not understand how people in this country could be allowed to go hungry. He was very compassionate toward their plight and wanted to help them. More than just feeding them and befriending them, he wanted to help them improve their lives.
He also talked a lot about the upheaval in South Africa and how he'd like to go over there and help end it.
McCandless showed a lot of compassion to people whom he believed really needed his help. On the other hand, he lacked all compassion for those he believed had money and wealth.
Actually, I noticed that the original question (contained, although misspelled, in the url) specifically requests specifics from Chapter 11 of Into the Wild, so I thought it would be fun to focus fully on that chapter. In Chapter 11, Chris' compassion is his focus in high school.
In high school, Chris becomes a big fan of the underdog in society, especially the poor of his own community. Through the observation of "the plight of the downtrodden," Chris comes to realize at this early age that money is the root of all evil (in his opinion). I love the way eNotes puts it:
In high school, Chris became passionately concerned with the plight of the downtrodden in his community and the world, and although he was a natural entrepreneur, he came to believe that wealth was corrupting and evil.
Personally, (and even though it's not in Chapter 11) I love the idea that Chris refused to join a fraternity in college as well. This suggests that fraternal organizations are less about brotherhood than they are about paying for friends. A similar pressure is released when Chris refuses to give gifts to people or to accept them.
In short, Chris McCandless has a real sense of justice that can be seen in Chapter 11 (as well as other places throughout the novel).