I think the plant itself was a detail. He was likely to hurt himself one way or the other. He was cut off from civilization or help, he didn't know about relatively nearby crossings (having "ditched the map" on purpose) and he had no survival skills.

He could have poisoned himself with something else, or been attacked by a wild animal, or treaded on some treacherous ground and broken a leg, all of this outside the reach of human help, and he did this on purpose.

> "Instead, I will walk into the wild"

But this is what I don't get. "I will commit suicide by walking into the wild without resources or knowledge", what's admirable about that? He was basically playing Russian Roulette! I mean, I get that suicide itself is the stuff of romantic ideation, maybe that's the thing?