Due to his arrogance/confidence, Beowulf wanted to kill Grendel without any armor(by convention even his clothes) or any weapon. He wanted to prove that he can kill Grendel on equal footing, that is to kill him only with his brute strength. The creature was known to rip men apart only with it's bare hands- and Beowulf wanted to show that he is stronger than Grendel even without armory
