beowulf must battle grendel with bare hands because?

Well, there are a couple of reasons here. First of all Grendel had this magical enchantment skin that was impervious to weapons. When you are that ugly, you need something going for you. Another reason was that Beowulf wanted to fight Grendel on his own terms. You know, man to monster, may the best man/monster win, "Unarmed he'll face me, if face me he dares".