Consider the convenience of Hygelac's death in Beowulf, and then his son's, which clears the way for Beowulf to feel he can legitimately assume the throne. Is this a plot device, or is there something more going on here?
The text remains remarkably silent about the precise nature of the fate of Hygelac and his son, Heardred. Both deaths are mentioned in two lines, with very little explanation about the details:
A lot was to happen in later days
in the fury of battle. Hygelac fell
and the shelter of Heardred's shield proved useless
against the fierce aggression of the Shylfings...
What is clear is that when Heardred was "cut down" by the Shylfings, the kingdom became Beowulf's. It is clear that this is largely a plot device that allows Beowulf, the undisputed hero of the text, to gain not just the glory and fame that he deserves, but then also the status he has earned through being king. After all, it is going to be very difficult for any other character to compete with Beowulf in terms of military prowess given all that Beowulf has accomplished. However, at the same time, it is possible to view this as more than a plot device. In Anglo-Saxon times, being a King ultimately meant responsibility for your people, rather than being a hero, which meant trying to make a name for yourself through courting danger. This change in role that Beowulf undergoes perhaps can be used to question his later actions in going to fight the dragon himself and thereby leaving his people defenceless and vulnerable. It suggests that the old heroic spirit in Beowulf has not died, and could be an implicit criticism of his inability to put the needs of his people above that spirit. His rise to kingship is therefore very interesting in terms of how it is used to develop the character of Beowulf and our understanding of his later actions.