Don't think this is the part people romanticize, I think it's the whole "I think careers are a 20th century invention and I don't think I want one" and the "Instead, I will walk into the wild" that does it for people.
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?
I don't think it's the suicide aspect, and - maybe besides the point - personally I don't think the guy was trying to die (not anymore or less than a freeclimber).
Maybe the Webster 1913 definition for "romantic" can help shed a light:
> Characterized by strangeness or variety; suggestive of adventure; suited to romance; wild; picturesque; -- applied to scenery; as, a romantic landscape.
I know what "romantic" means. I don't think McCandless was a romantic; I think he was reckless. His romanticism was entirely artificial -- Krakauer's opinion that he "threw away the map" on purpose since there are no really unexplored lands anymore hints at this -- and he didn't need to live this rough life, he strived to put himself in harm's way. Someone who crosses the highway with their eyes closes is not a romantic, they are simply stupid.
And like I said in another comment, I think he completely misread Jack London and failed to learn the lesson from that writer: that wilderness is ruthless and will kill you the minute you let your guard down, or simply out of bad luck.
Zooming out a bit, you could also say he died because he churned the philosophy of using the aids and crutches of civilization to maximize his hedonistic well-being at all cost - which is maybe a more interesting and ambiguous point to discuss than whether maps are useful ;)
Many herbivores are poisoned only when desperated to find alternative sources of food. He probably tried and failed to find other sources of food, so he ate legumes, that caused serious damage in their nervous system, and left him unable to walk and hunt anymore