The reason was him refusing to bring a map, which would have shown a bridge over the river less than a mile away.
And not scouting out his environment either, or even trying a detour along the river, or improvising a raft…

He also ate plants he didn’t know, which were poisonous and shut down his digestive tract.
But that was all after he made his fatal mistake, which was staying put when his actual food ran out instead of just walking back.

He wasn’t a dumbass, he was just suicidal.
He first left his privileged life, then his family and friends, then society. Typical steps of a suicidal person preparing for the end.
If he actually wanted to keep living, he could have left the bus and simply walked back, 3 days before his provisions ran out (it takes 6 hours to get to the trailhead).
But he stayed, even though he didn’t have the skills to live off the land. He accepted death, until it came close, then he regretted his decision and tried to get out but it was too late.

How do you manage to starve in a heated school bus literally on a trail, a few hours walk from a tourist lodge in the direction you came from?

It was made a capital offence by law in 1602 by the Edo government.

Previous laws regarding murder didn’t cover it?