In the Old English poem Beowulf, why does Beowulf face Grendel without a weapon?
There are at least three reasons for Beowulf's decision to face Grendel without a weapon--one based on circumstance; one based on what the poet tells us about a weapon's effectiveness; and one based on Beowulf's desire for fame.
First, as Grendel attacks Heorot (with Beowulf and his men asleep but anticipating an attack), Grendel immediately seizes one of Beowulf's men, rips him to shreds, and devours him "feet and hands" (l. 45). Next, Grendel moves to an adjacent warrior:
Nearer he stepped, and/grasped by the hand the great-hearted fighter [Beowulf] where he lay; Beowulf parried with his hand; he took hold on him quickly. . . . (ll. 45-48)
Grendel has fatefully grabbed the one warrior who is capable of defeating him, and using a defensive move based on circumstance, Beowulf, who probably has no room to maneuver with a weapon, first avoids Grendel's grasp and then grabs Grendel himself: "he took hold on him quickly/with fierce purpose and pinned his arm." (ll.48-49)
Second, as the struggle between Beowulf and Grendel continues, the poet tells us that Beowulf's warriors, in an effort to help protect him from Grendel, drew their weapons. Unfortunately, according to the poet, Beowulf's warriors
. . . didn't realize that no war-blades . . . could even scathe that sinful wrecker;/for he'd case spells against all/edged weapons whatever. (ll.801-805)
We learn here that one of the reasons for Grendel's success over the years is his invulnerability to the weapons of mankind. The poet subtly implies that only a man of Beowulf's strength and stature, using only his physical strength, has any hope of defeating Grendel.
The third, and perhaps most important reason, is cultural. Beowulf, one of whose goals is to seek personal glory, understands that defeating Grendel with only his physical prowess is crucial to achieving the greatest amount of fame in this contest. Using a weapon--assuming a sword or axe would be effective--to defeat Grendel would not lead to the same level of fame because Beowulf's detractors (Unferth, for example) could claim that Beowulf's success was the result of using a weapon against an unarmed (no pun intended) monster. In this contest, the odds are equalized by the fact that neither fighter is armed with a weapon. Beowulf's fame in the battle, therefore, is enhanced by his decision not to use a weapon. And fame is the goal.