Question: why does hrunting fail beowulf?
Answer: Hrunting fails Beowulf because it carries Unferth's stain: it belongs to that society of outcasts of which Unferth, Grendel, and Grendel's mother are members. If Beowulf were to wield Hrunting successfully, it would point to some character flaw of his own, associating him with that same group of sinners.