The answer to this question can be found in the final chapter of this book, Chapter Twelve, which details the long-awaited battle between Grendel and Beowulf and what happens as a result. One of the great aspects of this book is that it re-tells a tale from the perspective of the character traditionally thought of as being the evil character. Grendel's character and personality dominates this story and allows us to see things from his point of view, which makes us feel sympathy for him. Having for so long thought that he was invulnerable, Grendel is therefore very surprised when he attacks Beowulf and finds that Beowulf is much stronger than him. The battle is certainly won decidedly when Beowulf rips off Grendel's arm. Note how Grendel reports this event:
The room goes suddenly white, as if struck by lightning. I stare down, amazed. He has torn off my arm at the shoulder! Blood pours down where the limb was. I cry, I bawl like a baby.
Grendel, wounded mortally, is left to flee Hrothgar's home and return to his cave to die in agony, shouting out his defiance against an uncaring world. Beowulf has triumphed over Grendel.
Beowulf became the king but was easily seduced by Grendel's mother.