Because you are discussing a nonfiction work in which real people's deaths are portrayed, most of the major decisions made in the work are life or death decisions. Here are several:
1. Krakauer states that there are many inexperienced climbers on Mt. Everest during this climb, and as his team leader makes the decision to reach the summit, Krakauer wonders if this is a wise move. His question comes from the news that high winds and a storm are approaching, and since it takes at least 12 hours to reach the summit from the last camp, the author is skeptical that they will have time to summit and get down safely. As readers, we know, of course, that this ends up being the most costly decision made in Into Thin Air.
2. Guides and other climbers must make the decision to stay with stranded climbers. Rob Hall, a guide, heroically stays with one of his climbers, and when the climber dies, Hall does not have enough oxygen or orientation to come down the mountain. He ends up giving up his own life. Additionally, as climbers pass Beck Weathers lying in the snow, they decide that he is too far gone to try to help him back to camp; some even assume that he is already dead. Miraculously, Weathers does blindly make it to camp and doesn't blame others for leaving him, but one wonders if the climbers question their other decisions to leave behind people on the mountain.
These are the most significant choices portrayed in the book; but of course, you could also go back to climbers' decisions in the first place to attempt to conquer Mount Everest.