Poetic justice is a literary device where virtue is rewarded or vice is punished. Often, irony is involved or a twist of fate related to the actions of one of the characters. In this short story, Durante destroys the water tanks on Tony's ranch in an effort to protect himself from the posse that is pursuing him. He knows that when the posse gets to Tony's ranch, there will be no water for them, and they will be unable to pursue him. He runs off to the desert with a canteen that he unwisely let Tony fill for him. Tony puts wine in the canteen, so that in the middle of the desert, Durante has nothing to drink but wine, which is worse than drinking nothing. He ultimately dies of thirst.
The irony is that he thinks he has destroyed the water supply for the posse and only he possesses water to live, but in reality, his cruel actions have led to his own demise because by destroying Tony's water, this led Tony to get even with Durante. Tony and Durante were friends. If Durante had not shot up the water tanks on Tony's property, Tony most likely would not have put wine in the canteen. So Durante's cruelty backfired on him, and this is the poetic justice.