I want to quote at length from the article @spimon linked, because it's great, but quite long, and I want to highlight some sections for people who don't have time to read it all. It's very well written and reasoned, hitting all the important points.
What Chris did is common for wilderness survivalists today, who typically “survive” on negative calorie budgets, steadily losing weight. The only difference is that their excursions are normally of less than a month’s duration, and they simply gain back the lost weight after returning to civilization.
- Krakauer is deluding himself:
Having Chris die from a poisonous plant that could even fool “experts” makes him seem less foolish and overconfident than if he died by simple starvation. Krakauer’s incongruous interpretation of the evidence seems to be based on this desire to preserve a more positive image of McCandless—both for the readers and for himself.
- The derision of Chris's critics is unjustified:
Although Chris made serious and egregious mistakes, this is not a sensible reason to become furious at him or about what he did. The impulsive disparagement levied toward Chris displays the insecurities of a kind of redneck found in every rural district—one who feels deeply threatened by those who do things that he would not dream of trying and can’t understand. Only on the surface is this criticism about his fatal mistakes. Chris’s death verifies his critics’ self-image as rugged frontierspeople, and renders him a defenseless target.
When people say that Chris’s adventure was pitiful and insignificant, and imply that “Alaskans do that kind of stuff all the time,” they are kidding themselves. What they actually mean is that Alaskans go into the bush with snowmobiles or ATVs, lots of gear, and ample food supplies; why couldn’t Chris just do the same? This is as irrelevant and hollow as mocking a marathon runner because you can get to the finish line faster in your car. There is nothing inherently moronic about what Chris tried to do; he just failed. No person who has the ability to successfully do what Chris attempted would detest him for trying.
If you're at all interested in the subject, there's quite a bit more there, in particular a great deal of wise advice on foraging.