While we can never know about Chris McCandless's mindset and precise circumstances at the time of his death with certainty, little to no evidence would support the notion that he committed suicide.
The evidence we do have includes his journals/marginalia, the impressions he left on those who knew him, and his autopsy.
His writing and daily log do not suggest any suicidal thoughts or tendencies. In fact, on the contrary, he notes in his journal on Day 94, "Extremely weak. Fault of potato seeds. Much trouble just to stand up. Starving. Great Jeopardy." One can assume that he believed he was in "great jeopardy" of dying, and his wording suggests foreboding and apprehension about this, as opposed to excitement or relief.
On Day 100 he writes, "DAY 100! MADE IT! But in worse condition of life. Death looms as serious threat. Too weak to walk out, have literally become trapped in the wild -- No game." Again, his choice of words, "trapped," "serious threat," suggests that death was not his aim.
Additionally, the hikers and locals who discovered McCandless's body saw a note he left on the abandoned bus in which he was staying that read,
"Attention Possible Visitors. 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 ?"
His journals also reveal that McCandless decided to head back to civilization but was thwarted by the fact that the trail was blocked by a then-swollen Teklanika River (which had been much lower in the spring when McCandless arrived).
Turning to McCandless's relationships with others, he had made plans with various friends of his for the other side of his Alaskan adventure. Specifically, he expressed an intention to return and work at Wayne Westerberg's grain elevator in Carthage, South Dakota.
Finally, McCandless's autopsy has led most to believe his cause of death was a combination of starvation and poisoning due to the long-term ingestion of a certain breed of toxic potato seed. The coroners determined that McCandless weighed only sixty-seven pounds at the time of his death, his body containing no subcutaneous fat, and chemists found that the potato seeds he'd been eating indeed contained a potentially lethal dose of a neurotoxin called beta-N-oxalyl-L-alpha-beta diaminopropionic acid (ODAP).