I do not believe that Chris McCandless was mentally ill or suicidal. None of the journals that Krakauer used in the novel indicate that McCandless was suicidal. Additionally, McCandless left a note in the bus where he died that is begging for someone to give him aid and save his life. If he was suicidal, McCandless would not have written that note.
S.O.S. I NEED YOUR HELP. I AM INJURED, NEAR DEATH, AND TOO WEAK TO HIKE OUT OF HERE I AM ALL ALONE, THIS IS NO JOKE. IN THE NAME OF GOD, PLEASE REMAIN TO SAVE ME. I AM OUT COLLECTING BERRIES CLOSE BY AND SHALL RETURN THIS EVENING. THANK YOU, CHRIS MCCANDLESS. AUGUST?
McCandless loved life. He loved exploring and being free to go where he wanted to go when he wanted to go there. No, I do not believe that he was suicidal.
I do not believe he was mentally ill either. Sure, he loved going his own way, but that doesn't mean he's mentally unstable. I would say that he is fiercely independent and a loner, but that doesn't make him mentally ill. Furthermore, every person that McCandless interacted with spoke well of him. Krakauer doesn't write that people like Westerberg found McCandless unstable. It was quite the contrary. Most people wanted McCandless to stay longer, because they enjoyed his company and were surprised at how intelligent he was.
I don't believe there is any evidence in the novel, Into the Wild, or in Chris McCandless' own diary entries that he was suicidal. He enjoyed a love of life and adventure, and there is evidence that he planned to eventually settle down and raise a family after he had experienced his planned and unplanned excursions around America. One might think that many of his decisions were caused by mental instability, but he was above all a young man with a desire to experience a nomadic existence without parental intrusion. His tragic end was undoubtedly an unexpected one, but it was primarily due to a lack of preparation and some bad luck rather than mental disease.