In Homer's The Odyssey, Melanthius arms the suitors—who have disrespected Odysseus' family, home and servants—and it is definitely considered by Odysseus as treason. The suitors are Odysseus' enemies, waiting like vultures to take his "widow," home and property by force.
In turning his back on Odysseus, and supporting the suitors, Melanthius not only betrays the master of the house, but throws in his lot with the suitors who Odysseus has promised to kill.
As they sit in the great hall, the suitors have been separated from their weapons and armor. When Odysseus begins his attack, they have nothing with which to defend themselves. Melanthius (unbeknownst to Odysseus), takes weapons from where they have been stored and starts to outfit the suitors. Odysseus has a moment of concern when the suitors begin to arm themselves, noting that someone must be helping them.
On this the goatherd Melanthius went by back passages to the store room of Ulysses, house. There he chose twelve shields, with as many helmets and spears, and brought them back as fast as he could to give them to the suitors. Ulysses' heart began to fail him when he saw the suitors putting on their armour and brandishing their spears. He saw the greatness of the danger, and said to Telemachus, 'Some one of the women inside is helping the suitors against us, or it may be Melanthius.'
Odysseus punishes all those who consorted with the suitors, this includes Melanthius and any women who slept with the men vying for Penelope's hand. Melanthius is punished for his part, first hung from the rafters by the neck to suffer. Euryclea reports regarding the serving women's behavior: they slept with the suitors. Odysseus orders that the women first be made to help dispose of the dead, and then be killed themselves.
When the women have been dealt with, Odysseus returns his attention to Melanthius, who is mutilated for his treasonous acts.
As for Melanthius, they took him through the cloister into the inner court. There they cut off his nose and his ears; they drew out his vitals and gave them to the dogs raw, and then in their fury they cut off his hands and his feet.
The actions that Melanthius takes against Odysseus and his household are considered traitorous, and Odysseus takes his revenge on those who are guilty.