Full spoilers follow.
The map. The document stolen from the Japanese official in the opening train robbery is believed to lead to a buried Qing dynasty treasure. Its true value is that everyone believes in it: the Japanese Army wants it, Korean independence fighters want it, Manchurian bandits want it, and the Bad is hired to retrieve it. It functions as a classic MacGuffin, and the film ultimately treats it as one.
The personal history. The engine of the plot is not the treasure but the hidden connection between the Weird and the Bad. Park Chang-yi's pursuit of Yoon Tae-goo is revealed to be rooted in a past encounter that humiliated him — the disfigurement he carries and the reputation he guards. The bumbling thief who claims to be nobody is in fact the one man who beat him, and Chang-yi's obsession with primacy makes killing him a matter of identity rather than contract.
The climax. All three men converge in a three-way standoff staged as a direct homage to the cemetery duel of Leone's The Good, the Bad and the Ugly. The circling tension resolves in near-simultaneous gunfire; all three go down bleeding.
The treasure revealed. Standing over the supposed treasure site, bullet-riddled, the men trigger the film's real twist: the ground bursts open and what erupts is not gold but oil. The "treasure" of the coming century is black liquid, and it is worthless to dying men in a desert. The wind then tears the map from their hands and carries it away across the dunes — the object that cost dozens of lives simply leaves, unclaimed.
The two endings. The international cut (about 129 minutes, and the director's preferred version) ends on that bleak, elegiac note. The Korean cut (about 136 minutes) continues: Tae-goo rises, revealing that he had a thick metal sheet concealed beneath his quilted jacket, and limps over to Chang-yi's body. Desecrating the corpse, he discovers diamonds in the dead man's pocket and giggles with delight — before realizing he is completely surrounded by the Japanese Army. The trickster survives, gets his fortune, and is immediately trapped by the imperial power that has been the film's true horizon all along.
What becomes clear afterward. Retrospectively, the comedy of the Weird's survival is not accident but design — he lives because he is the only one not fighting for status. The Good's early aphorism about chasing and being chased becomes a plot summary. And the title's labels invert: the "Good" was a bounty hunter working for money, the "Bad" was a man nursing an old wound, and the "Weird" — the coward and grave-robber — is the only one who ever wanted merely to live.