How can I argue that, "The affect the Pardoner has on us, as Chaucer describes him, is at once, comic, contemptible, and sinister"?
Certainly, it is somewhat contemptible that the sanctimonious hypocrite, the Pardoner, is a man of the Church who himself sells phony relics and makes the pilgrims wait for his tale while he drinks. And, it is ironic that he proposes to tell a moral tale about greed.
Further, here are how the comic, contemptible, and sinister play out in the Pardoner's Tale:
The "old, old fellow" that the three rioters encounter in their venture to kill Death is obviously the personification of this malign force itself. "Not even Death, alas, will take my life" is a sinister statement as it threatens, ironically, the three miscreants who unknowingly find what they seek to kill.
The three "rioters" are contemptible in their speech to the old fellow that they meet, telling him he should die. Speaking "roughly" to him, calling him "an old fool," they order him to "Give place!" The man described as the proudest says,
"Why are you all wrapped up except your face?
Why live so long? Isn't it time to die?" (ll.58-59)
After they curse him with God's name and the sacraments, the old man tells them that if they seek Death, they can find him under a tree. When they abandon the old man and rush to find Death, they quickly forget their purpose when they find a "pile of golden florins" lying on the ground.
But, of course, the most contemptible acts of the three are their treacherous efforts to keep as much of the gold for themselves as two plot the death of the youngest, and he plots the death of the two who wait under the tree when he is sent to town.
The elements of irony provide some comic relief from this moral tale. That the three men are talking to Death himself and they belittle him is ironically amusing. That they forget all about Death when they discover the gold, is laughable. And, at the end, that the foolish two who kill the youngest drink the wine, not ever considering that he may be as greedy as they is also darkly comic.