McCandless starved to death. Why he starved to death is not settled (and probably never will be), and a poison which prevented his body from metabolizing food is one suspected contributing factor: https://en.wikipedia.org/wiki/Chris_McCandless#Death

I thought the berries just weakened him rather than killed him outright. In his weakened state he was unable to go forage for food and due to his malnourished state (especially lack of fats) could have died from the cold just as easy. I need to re-read the book though it has been a long time, I really enjoyed it and I remember having a feeling of understanding and agreeing with his way of life thanks to problems I had at the time with major depression. I remember I even planned to leave everything behind and go and head to Alaska to try and see what he saw before I probably died myself.

I think the effect paralyzed him and then he died of starvation.

I believe he ate such things because he was starving though.

This is incorrect. Jon Krakauer who wrote Into the Wild pulled this out of his ass. The dude just starved cuz he had no idea what he was doing. He was a pretty shitty person all around.