Meanwhile the daughter of Icarius, wise Penelope, had had a rich seat placed for her facing the court and cloisters, so that she could hear what everyone was saying. The dinner indeed had been prepared amid much merriment; it had been both good and abundant, for they had sacrificed many victims; but the supper was yet to come, and nothing can be conceived more gruesome than the meal which a goddess and a brave man were soon to lay before them—for they had brought their doom upon themselves. (Last lines of Book XX)
The suitors have been in Odysseus' house, feasting on Odysseus' food and mistreating his maidservants and his son. In addition to this, they have abused a guest in Odysseus' house. This incident of the suitors mistreating a guest (and an old, poor, and infirm one at that, or so the suitors think) is considered an offense against that gods in addition to being an insult to the (they think absent) Odysseus, Penelope, and Telemachus. The Greeks of Homer's world held the guest-host relationship as particularly sacred, and the insult was as great to the gods as it was to the guest and the host. The suitors have committed a rather serious sacrilege.
One suitor, Theoclymenus, however, realizes that the other suitors are behaving in an unacceptably outrageous manner, and wants no part of it. The suitors, after having thrown a cow hoof at Odysseus, have been overtaken with uncontrollable laughter. This raucous scene disgusts Theoclymenus, and he wisely leaves the house. He does not know it then, but this saves his life.
So this is a scene of feasting and debauchery, and the suitors are very arrogant and full of themselves. They do not know what is to come, however, and the irony is that the "victims" (the many animals that were killed for the feast) are mentioned among the talk of the suitors' destruction. The suitors do no know it, but they will soon be cut down in this hall just like the sacrificed animals they had lately killed. It is a dark irony, full of foreshadowing of doom. This kind of grim comparison is typical of Homer, who relished the downfall of the unrighteous in a world in which justice was so often not served. Odysseus has lost many years of his life, and his entire crew, on his epic journey home; Homer is preparing us for Odysseus' final triumph. That it is a bloody slaughter, and is a less than ideal homecoming, is not lost on Homer. Penelope is watching all of this outrageous revelry, and she does not yet know that her husband is the abused beggar-guest, so the irony is sharpened.