Final answer:
Beowulf begins to insult Unferth between lines 315-323, calling out Unferth's cowardice and fratricide, thereby undermining his reputation as a warrior.
Step-by-step explanation:
The discussion between Beowulf and Unferth in the epic poem starts to take a confrontational turn within lines 315-323. At this point in the narrative, Beowulf begins to insult Unferth by calling attention to his lack of bravery and his failure to stop Grendel. Beowulf highlights Unferth's inefficacy contrasted with his own heroic capabilities, suggesting that if Unferth were as courageous as he claims, Grendel's terror would have already been addressed. Beowulf also points out Unferth's past actions, namely the killing of his brothers, which further undermines Unferth's reputation and status among the Danes.
By insulting Unferth, Beowulf is not only defending his own honor but is also questioning Unferth's reputation and status as a warrior. This verbal interaction is characteristic of the heroic culture depicted in the poem, where warriors boast of their achievements and challenge each other's bravery and honor.