'Thin Air's a great book. Most of the hostility toward Jon started from people on the expedition, or friends and families thereof, or others in the 'climbing community' in general.
I think the basic reason's pretty simple: JK thoroughly exposes the incredible stupidity of the whole endeavor from every angle. The vanity, hypocrisy, shallowness, dishonesty, and flat-out stupidity, and *incredible* irresponsibility that underlie every aspect of the Everest 'business' are really laid bare in his book. (That's a really big part of why it's such a good book.)
None of those people really want to admit any of this about themselves, or their friends and family.
So they lashed back hard at Jon, and looked hard for particular things to hang on him, like the fact that he initially mis-identified one climber because of the hypoxia, exhaustion, and chaos.
(Really not his fault, totally understandable, and he seemed to feel horrible about it, went to great lengths to correct it and apologize, endlessly beat himself up about it --even though his mistake didn't directly harm anyone, and the actions of many others certainly did-- but lots of people acted as if he had personally killed somebody.)
I really think most of it's basically that simple.
It's a great book. Too bad the movie was an absolute piece of trash. (Wow, was *that* disappointing.)