While the Pardoner himself would like his listeners to draw the moral that greed leads to destruction, Chaucer implies a more subtle lesson: beware of hypocrisy.
The Pardoner's tale is about three young revelers. These young men spend their time drinking and carousing, and they become extremely indignant when they learn that Death has carried off another friend of theirs. They decide they will take their revenge on Death.
The three young men meet an old man who tells them that they can find Death under a tree just over yonder. When they arrive at the spot, however, they find eight bushels of gold coins. They decide to wait until nightfall to bring the coins into town under the cover of darkness, and one young man goes off to get food and drink. Meanwhile, the other two plot their friend's demise so they don't have to share the gold with him. He, too, decides he would like the gold all for himself and poisons the bottles of wine that he brings back. The two young men jump on their friend when he returns and kill him, but then they drink the poisoned wine and die too. Indeed, they have found Death under that tree, and their greed has led to their destruction.
But there is more to the story than that. The Pardoner himself is a greedy, plotting, dishonest scoundrel who enjoys an abundance of good food and drink, who carries around fake relics and charges people to venerate them, and who generally does the exact opposite of what he preaches. Of course, he only preaches for money anyway. He is, in essence, no different from the three young men in his tale. He is a hypocrite, and he is a man to beware of, for hypocrites may sound good, but they are just as dangerous and destructive as greed.