Envy feeds upon itself and is, indeed, one of the seven "deadly sins," the Pardoner implies in his tale delivered to the other pilgrims.
Ironically, the Pardoner tells a moral tale. For, he boasts of his unconscionable behavior as he sells indulgences and pardons to people, deceiving them by selling pig's bones as purportedly the relics of saints, telling them that a glove works magic if they put their hands into, and other hoaxes. Thus, he exemplifies his own greed that exploits the envy of others.
The Pardoner's Tale is an exemplum, or a moral anecdote meant to illustrate a theme, which the Pardoner states is "Radix malorum est cupiditas" which is Latin for "The root of all evil is greed." This fictitious tale of the Pardoner is about three riotous young men who learn that one of their friends was slain by Death. Vowing to find Death and seek revenge, they set out, but instead encounter an old man who tells them he has met Death and left him in a grove where he sits beneath a oak tree, having expressed his intention to remain there. When they search for Death under this particular tree, they find, instead,
hoard of golden coins, so fine and round...
And as many bushels, it seemed to them as seven
But, "the worst of the three" cautions the others that if they are seen toting this hoard into town, they will be believed thieves; so, they should wait until the cover of night. Without anything to eat or drink, the men decide to draw lots to see who will go to town for provisions. It is the youngest of the three who sets off for town, but as he treks along, he considers how much more he could have if the others were not alive. In town, then, he purchases poison and puts it into the two bottles of wine intended for the others and returns with the provisions. However, the other two, equally as envious as the youngest, have plotted to kill him; therefore, when he arrives, they set upon him and slay him.
"And now let's sit and drink, and make us merry,
And afterward we'll dig a hole and bury him."
Reaching for the wine, one of the envious murderers chose a bottle that has been poisoned; in their merriment at their good fortune, they pass the bottle back and forth, until they eventually succumb to the poison and die, a fair retribution for their desire for what they have not deserved.