Which Swedish king kills Heardred in Beowulf’s backstory? Onela does. Onela is a Swedish king of the Scylfing dynasty and the son of Ongentheow. He is also the brother of Ohthere and the uncle of Eadgils and Eanmund.
The episode appears in Beowulf’s account of the Swedish-Geatish conflict. Eanmund and Eadgils, sons of Ohthere, flee to the Geatish court after a dispute with their uncle Onela. Onela attacks the Geats, and during that campaign his champion Weohstan kills Eanmund. Heardred, Hygelac’s successor as king of the Geats, is also killed. Onela then returns to Sweden, while Beowulf succeeds Heardred as king.
The story does not end there. Beowulf later supports Eadgils with warriors and weapons against Onela. Eadgils wins that struggle and kills his uncle, a detail that links the poem’s Swedish feud to later Scandinavian traditions about Aðils and Áli. A common mix-up is identifying Ongentheow as Heardred’s killer; Ongentheow belongs to an earlier generation and is Onela’s father.