The relationship between the monstrous and the human in Odyssey is one which showcases there is a thin line between being human and being monstrous.
There are numerous instances where this is evidence such as the scene where Circe turned the men of Odysseus into swine, the strangely human like characteristics of the Cyclops Polyphemus as well as the way in which the crew of Odysseus encountered the cannibalistic Laestrygonians.
In each stage of the adventure readers are introduced to an ever increasing similarity between what is monstrous and what is man to the point that the line between the two blurs resulting in actions of Odysseus at Ithaca which can be considered monstrous but in fact is a purely human action as portrayed by the story.
The Cyclops Polyphemus
One of the first encounters of Odysseus with something that was truly monstrous was the Cyclops Polyphemus who ate some of the men of Odysseus and could be considered a truly frightening site with his single eye and enormous size. However, it is interesting to note that despite Polyphemus being a monster he was quite articulate and was able to carry on a conversation not only with Odysseus but with Poseidon himself.
His act of complaining to his father (such as when a child would complain to this own father if he/she were being bullied) is very human like and is the first instance where we seeing a blurring of the lines between man and monster.
The Witch Circe
The second encounter of Odysseus with the supernatural came in the form of Circe turning some of his men into swine (i.e. pigs). In this instance in the story it is implied that men are no different than beasts as implied by Circe through her dialogue in the story wherein she has had bad experiences with men.
It is due to this dialogue and her transformation of the men of Odysseus that we see another blurring of the lines between man and monster.
Encounter with the cannibalistic Laestrygonians
Prior to landing on the island of Circe, Odysseus encounters the Laestrygonians who are mostly human yet are giant in size (i.e. being taller than the average person) these individuals turned out to be cannibals who destroyed the ships of Odysseus (except his own since it was hidden) and ate his men.
In this instance we see humans (albeit very tall humans) acting in a distinctly inhuman fashion. At this point of the story the line between what is human and what is monster is further blurred since we now see humans acting exactly like monsters yet having the external appearance of humans.
Slaying of the Suitors
Towards the end of the story we encounter Odysseus arriving at Ithaca and after a few scenes and tests slays all the suitors of his wife.
This particular scene is included in the blurring of the lines between man and monster since Odysseus in effect performs an act of brutal murder on several people. Yet, despite such an act, he is not punished in any way within the story and is even praised for his deed.
Based on what has been presented so far, it can be seen that in the odyssey the relationship between the monstrous and the human is one which showcases there is a thin line between being human and being monstrous wherein man can be just as monstrous as the monsters themselves.