Beowulf's behavior before and during his battle with Grendel is courageous, proud, and honorable. Beowulf, when he hears of Hrothgar's plight, doesn't hesitate to assemble a band of men to travel across the sea and take on Grendel, who threatens all of civilized human life with his attacks on the mead hall.

When Beowulf arrives at court, he shows his understanding of the proper forms of address, as well as a great deal of pride, when he tells Hrothgar and his men of his remarkable abilities in battle:

So every elder and experienced councilman
among my people supported my resolve
to come here to you, King Hrothgar,
because all knew of my awesome strength.
They had seen me boltered in the blood of enemies
when I battled and bound five beasts,
raided a troll-next and in the night-sea
slaughtered sea-brutes.

Beowulf is stating that he is an exceptional fighter. To add to the pressure on Beowulf, his story is discounted as bragging by Unferth, one of Hrothgar's men. Beowulf, whose integrity is called into doubt, now must prove himself.

Beowulf, however, is determined to behave with honor. Since he has heard that Grendel does not fight with weapons, he decides he will face the monster in hand-to-hand combat. Beowulf states,

the monster scorns
in his reckless way to use weapons;
therefore ... I hereby renounce
sword and the shelter of the broad shield,
the heavy war-board: hand-to-hand
is how it will be, a life-and-death
fight with the fiend. Whichever one death fells
must deem it a just judgment by God.

This may also be a practical decision, because the men who have tried to fight Grendel with weapons have not succeeded. Beowulf's statement also shows that he faithful to God, a contrast with the godless Grendel, and is an indication that he has some humility: whatever strengths Beowulf might possess, he knows it is still up to God to decide the outcome of the battle.

Beowulf is brave, resourceful, and confident in his battle with Grendel, which he wins.

Beowulf is an interesting character, in that we as the audience can never be certain of how far his self-confidence is justified, or of how the poet really feels about it. The word "ofermod," used to describe Beowulf, has been the subject of considerable debate: does this mean overconfidence, or arrogance? And is it a criticism?

Certainly, before his battle with Grendel, Beowulf takes part in the sort of heroic boast typical of the epic tradition when he tells Unferth the story of his previous battle with sea creatures. Unferth is less than impressed: he does not believe that Beowulf is truly as superhuman as he describes himself. However, Beowulf does seem to justify his words when he embarks upon his battle with Grendel. He is extremely confident, but this is because he knows that he has more strength than any other man and will have a better chance of defeating the monster Grendel than an ordinary human. Like Grendel and Grendel's mother, there is something supernatural about Beowulf, and so when he approaches Grendel, he does it boldly and bravely and does not seem to regret having volunteered to rid Hrothgar and his people of their marauding monster.

During his battle with Grendel, Beowulf commits himself to the task of destroying the monster, showing the same courage and self-confidence that he has earlier exhibited in his war of words with Unferth in the mead hall.

Beowulf shows himself to be a brave young man desperate to prove himself as a warrior. Despite his relative youth, he knows that he is different from other men, and that if anyone can defeat the blood-thirsty monster Grendel, it's him. Beowulf is also possessed with excess pride. Ultimately it will prove his undoing when, as king of Geatland, he will engage in one fight too many. But for now, Beowulf's hubris enables him to perform truly epic feats of daring, like swimming for five days and nights in icy seas while carrying a sword, and battling deadly sea monsters.

As well as arrogance and boastfulness—witness his outrageous disrespect of Unferth—Beowulf's excessive pride leads him to answer Hrothgar's SOS call without a moment's hesitation and head on over to Denmark to slay Grendel. Beowulf may be boastful and positively bursting at he seams with pride, but he's not like Achilles; he does have a wider sense of responsibility to others.

Beowulf's behavior is that of a typical hero in the epic tradition. First, we have Beowulf sailing to the aid of Hrothgar to reciprocate for Hrothgar's helping his father. This shows Beowulf to be of noble and responsible character, loyal to his family and embedded within a reciprocal network of social and military obligations. Beowulf engages in a battle of words with Unferth, and in his description of the swimming contest and vanquishing of the sea monster does two things, foreshadows his victory over Grendel (who is also a monster associated with the sea) and begins a classic pattern of the boasting in which heroes of oral epics typically indulge before a major battle. 

At the start of the battle, Beowulf remembers his boasts and uses them to grant him to courage to fight Grendel. Because no weapon can wound Grendel, Beowulf rips off the monster's arm, and afterwards it is displayed as a trophy. Beowulf then attends a banquet and is richly rewarded by Hrothgar.