How does Beowulf trap and kill Grendel? Beowulf trapped and killed Grendel by ripping his arm off. Little does Beowulf know that Grendel is not dead yet.
What drives Grendel to attack so many men at Herot, the mead hall? Because he is angry at the Danes.
Why does Beowulf hang Grendel's arm from the rafters of Herot? It's his price possession for defeating Grendel. It can show everyone the Beowulf has won the battle.