Well Beowulf dies after personally battling a Dragon that burns and destroys his lands and kingdom. Beowulf decides to fight the dragon, but finds that he is outclassed in the battle. His men (who he told to wait for him) become afraid and all of them except for on warrior named Wiglaf, flee. Beowulf is wounded in the battle and Wiglaf comes to help him, together they slay the dragon, with Beowulf dealing the killing stroke. Afterwards, Beowulf succumbs to the wounds he felt in battle.
So that is the reason for Beowulf's death, and the symbolism within the poem is meant to focus on the hero's stand against evil and his knowledge that, in failing to defeat evil, he will fail his people. Ultimately, he succeeds in slaying the dragon and triumphing over evil. However, he also fails because he is killed in doing so, and that means the threat of war and destruction for his people.
