I think that a good case can be made that Odysseus was justified in taking action against the suitors. Part of any justification of Odysseus's behavior resides in the dishonorable way that the suitors treat his home and its inhabitants. Homer shows honor as a critical aspect of Greek society. It is the reason why the war against Troy is fought. Honor is vitally important to the Homeric Classical setting. It becomes evident that the suitors bring a sense of dishonor to Odysseus's home. The fact that they overtake the residence without any regard for the man of the house itself is one example of dishonor. Additionally, the suitors vie for Penelope's hand in name only. The suitors are there for the prizes of Odysseus's home and show little in way of respect for the traditions and sense of decorum that Penelope displays. Penelope herself says as much in challenging the suitors to string Odysseus' bow:
Listen to me you suitors, who persist in abusing the hospitality of this house because its owner has been long absent, and without other pretext than that you want to marry me; this, then, being the prize that you are contending for, I will bring out the mighty bow of Odysseus... and quit this house of my lawful husband, so goodly, and so abounding in wealth.
Penelope's statement indicates behaviors that would justify Odysseus's anger. The persistence in "abusing the hospitality of this house" as well as the "pretext" of marriage can be combined with the "abounding in wealth" as independent reasons why Odysseus' anger is justified. In the larger sense, his anger is further substantiated by hearing his wife speak these words. There is a despondency present in Homer's depiction: Odysseus having to see his house reduced to an undignified state and seeing his beloved wife imploring the men who disrespect his home to simply listen to her. This instant could be seen as a justification of his brutality towards the suitors.
A potential argument against Odysseus's actions suggests that he must assume a more magnanimous stance that would reflect his position as a king. Odysseus should act "more regal" and refrain from violence. This argument would also suggest that he should not sink to the level of the suitors. Yet, it should be noted that Odysseus does not instantly slaughter the suitors. He approaches them, in kind and disguise, with a sense of dignity and aplomb. He even suggests a "way out" for the suitors:
Suitors of the illustrious queen, listen that I may speak even as I am minded. I appeal more especially to Eurymachus, and to Antinous who has just spoken with so much reason. Cease shooting for the present and leave the matter to the gods, but in the morning let heaven give victory to whom it will.
Odysseus accepts the abuse from the other suitors while he is in disguise. He does not retaliate. Yet, it becomes clear that the suitors only wish to usurp Odysseus, living off of his endeavors and the fruits of his labor. This is the highest amount of disrespect and dishonor. Once it becomes clear that there is no "philos" (Greek for love and respect) towards Odysseus, and little in way of loyalty, he strikes. Odysseus could not have re- asserted control in any other manner. He had to take action against the suitors, something that in the name of honor and loyalty he saw as completely justified.
The conflict within the Odyssey revolves around the theme of homecoming, which in Greek is "nostos." In order for Odysseus to return home, he must purge his household of the suitors. Thus, the slaughter that occur near the end of the Odyssey is justified.
Many suitors had taken residence within the walls of Odysseus' palace, and they attempted to persuade his wife Penelope to marry them. In the duration of their stay, they depleted Odysseus' wealth and disrespected both Telemachus and Penelope during the king of Ithaca's absence. Their recklessness is paralleled in the Odyssey by Odyssues' men who consume Helios' cattle. Furthermore, it was decreed by Poseidon, who acts of vengeance against Odysseus serve as a kind of justice for the harm he has done to his son, the cylops, that if Odysseus were to return home, he would do so without any companions. Thus, just as his crew from Troy perished by divine decree, so too did the suitors.
Athena prepares both Telemachus and Odysseus for the battle that will take place between them within the walls of their home. She even fights beside them in the form of Mentor. Athena ensures that Odysseus will not meet the same dreadful fate of Aegisthus, who perished in his household after returning victorious from war due to the infidelity of his wife. This tale is recounted in the Oresteia, in which Athena is a driving force for realizing justice as well.
Finally, in the final book of the Odyssey, Odysseus, Laertes, and Telemachus must face the people of Ithaca and the relatives of the suitors. Rather than allowing the slaughter to spread in a battle beyond Odysseus' household, Athena ends the violence, commanding that they return to their homes and live in peace as justice has already been served.