To put it simply, Everest's summit is the Holy Grail. As we see in Into Thin Air, climbers drop tens of thousands of dollars, undergo months of preparation, and endure Saw-levels of physical suffering just for a shot at reaching this tiny patch of land at the top of the planet.
Unfortunately, this leads many to fall prey to nasty little ailment dubbed "Summit Fever." Summit Fever is an obsession with reaching the top of a mountain at all costs, often at the expense of your own physical wellbeing or that of your comrades.
We see this in effect on the north side of Everest when a group of Japanese climbers passes several dying men on their way to the summit. As the group trudges by, "no words were passed" and "no water, food, or oxygen exchanged hands" (18.10). Although most of the folks depicted in Into Thin Air aren't quite so callous, everyone is irrationally driven to reach the summit to some extent.
Although the summit might seem like the finish line, the truth is that it's only the halfway point—you still have to climb back down. So not only does Summit Fever cause climbers to compromise their morals, but it also leads some to exert all their effort on the way up, only to be left too exhausted to come back down. This is an irony perfectly expressed by Rob Hall himself: "With enough determination, any bloody idiot can get up this hill […] The trick is to get back down alive" (11.11). In short, if you see the summit as the prize, then you might lose track of the bigger picture: getting off Everest alive.