Beowulf's armor with the images of birds makes him into an avatar of a bird. Although he does not take flight, and although the armor is being removed at the end of the poem, he nonetheless becomes associated with a bird. He is literally a creature of the air, suggesting that his role, like a bird's role, is to fly above the fights.
As the dragon falls, it pushes the sea up onto the beach, and Beowulf drags it out into the open air, and then it sinks out of sight. The dragon had been a dwelling creature; in this act of violence, Beowulf is casting out the darkness of the dragon's lair, reducing it, treating it as less than human.
He associated with the dragon because it is the sort of creature that he is. Growing up as a prince in Heorot, Beowulf is not a true warrior but a prince; the dragon was not something that he laid eyes on except in the occasional fight. The dragon is an unknown and frightening world into which he had entered in order to steal a treasure. He becomes a real warrior after his encounter with the dragon, and his unique quality of mind and heart, his personality, is defined by that encounter.
Nothing is more terrifying than the monster from the dark. Perhaps for that reason, the most terrifying part of Beowulf's fight with the dragon is not that he has wounded it, which is bad enough, but that at the end of the fight, the monster is, in effect, defeated.
When Beowulf reflects on Grendel's hag mother (who has not been mentioned previously) and on her advice to her son, he says that Grendel was destroyed by Grendel himself, not by the people of Heorot. d2c66b5586