Chaucer wrote The Canterbury Tales at a time period in England when the Church was extremely corrupt and the peasants were at the mercy of the Church. He wrote it as a means of exposing the Church's corruption. As a pardoner for the Church, meaning one who pardon's sins for payment, the pardoner is proven to be one of the most corrupt among the pilgrims, and "The Pardoner's Tale" exposes this corruption. Since "The Pardoner's Tale" represents the height of corruption among the pilgrims, we can see "The Pardoner's Tale" as somewhat of a climax, which one could see as a sort of finale. However, I would actually hesitate to call "The Pardoner's Tale" the true ending to The Canterbury Tales. The book actually ends first with "The Parson's Tale" and then with "Chaucer's Retraction."
The pardoner in "The Pardoner's Tale" clearly portrays Church corruption when he openly confesses to his deeds of acquiring money from gullible people in return for what he claims is his ability to pardon their sins. He evenly openly declares he cares nothing for the welfare of humanity and instead only cares about luxury and wealth. His tale perfectly illustrates both how much he is entrapped by money and greed and exactly what the consequences of greed are. In his tale, three drunken buddies go off to pursue a murderer they heard of called Death. They then learn that Death is actually a pile of gold by a tree. Appropriately, two of the three friends plot to murder the third in order to acquire all of the gold for themselves, while the third friend plots to murder the other two at the same time for the exact same reason. The end result is that the two friends successfully murder the third, but then die themselves after drinking the wine that had been poisoned by the now dead third friend. In other words, their greed for gold kills them all, which is exactly why the gold was given the name Death. In this story, the greed for gold also represents the Church's own greed for gold and its inevitable corruption on account of this greed, as seen through the pardoner. Since the tale perfectly represents the heart of Chaucer's point to illustrate the corruption of humanity, especially the corruption of the Church, we can see how the tale represents a climax in the book. But the book wouldn't have a true ending unless we hear Chaucer's own statement of what humanity should be doing instead. "The Parson's Tale" has been interpreted as an "elaborate and authentic statement of the nature of sin and the importance of penitence" ("Overview: Background") In his tale, the rooster Chanticleer starts out by being boastful, prideful, and arrogant, just like the corrupt Church. But when he is captured by a fox, he easily tricks the fox into being boastful and prideful, thereby escaping the fox when he convinces the fox to boast about his capture. Since Chanticleer used his own character flaw to escape, we can easily see how the story portrays the importance of realizing your own character flaws and repenting, and then even using your flaws to your own advantage. Chaucer's purpose for writing The Canterbury Tales is further stated in the parson's final lines:
And now, good God, and if it by Thy will,
As says Lord Christ, so make us all good men
And bring us into His high bliss.
Hence, it is really "The Parson's Tale" that relays the final message and is therefore the real ending of the story, while "The Pardoner's Tale" merely clearly states the heart of the issue, Church corruption.