What do the rioters' actions toward one another represent?

I assume you mean in "The Pardoner's Tale." At the end, the rioters who have found the gold under a tree agree to split it. One man goes to town to buy wine for a celebration and then has it poisoned.  The other two decide to kill the "wine getter" with the intention of only splitting the money two ways.  They kill him when he returns; they then drink the win to celebrate their money and die because the wine is poisoned.  All then die from their greed showing that in the most basic way they did find death under the tree just as the old man had said.