Time passes, and Hygelac dies in battle with the Franks. His son Heardred rules after Hygelac, but he is also soon killed in battle. The throne comes to Beowulf, who rules as a great, wise, and prosperous king for fifty years. But then Beowulf's reign is disrupted by the appearance of a dragon.
Nothing is permanent. Time always wins. Beowulf, like Hrothgar, rules well for fifty years. But then trouble strikes, changing his situation when he's an old man.
The dragon guards an underground barrow full of treasure, which is accessible only by a secret passage. One day a slave, fleeing a beating, finds his way to the passage and sees the dragon. Despite his terror the man steals a cup from the treasure.
The dragon is the opposite of a good king, hoarding treasure instead of rewarding loyalty and building a society.
The narrator explains that this particular barrow was the treasure of a lost tribe. Long ago the last living man of the tribe placed his peoples' treasure in the barrow, since it was of no use to him.
Without a generous king to give the treasure and loyal warriors to earn it, the treasure is "useless."
The dragon discovered the treasure sometime later, and guarded it in peace for the three hundred years. But when the dragon wakes and notices the slave's footprints and the missing cup, it is filled with fury and bursts into the air to hunt for the man who stole the cup. Though the dragon fails to find the thief, it takes vengeance by ravaging the countryside at night. The dragon burns the land and buildings, including Beowulf's own mead-hall.
As a monster that represents the opposite of a generous king and therefore is a destroyer rather than creator of society, it should come as no surprise that the dragon would burn Beowulf's mead-hall.