So, you are referring to the 2007 film version of Beowulf, and not the original epic poem. In this film, there is a major difference in that Beowulf does not kill Grendal's mother; instead, when he travels into her lair, he finds that she is a beautiful temptress. She asks him to provide her with a son and promises him that he will be king. In the original text, of course, Beowulf defeats Grendal's mother, who is described as a "hell - dam" in this original text - a very different description than Angelina Jolie's sensuous version of her. He travels to her lair and the original text states:
So the Shieldings’ hero, hard-pressed and enraged,
Took a firm hold of the hilt and swung
The blade in an arc, a resolute blow
That bit into her neck bone
And severed it entirely, toppling the doomed
House of her flesh; she fell to the floor.
Clearly, there is no temptation on Beowulf's part in this version. He kills her, and all of Hereot rejoices. In the text version Beowulf is described as "the noblest of men." His behavior illustrates this nobility: when he arrives in Hereot, he states,
My one request
Is that you won’t refuse me, who have come this far, 430
The privilege of purifying Heorot,
In the text version, Beowulf is a heroic and noble man. Though he is sometimes a bit arrogant, this adds to his confidence, and he does admit his weakness when necessary - an example we see when he wears chain mail to fight Grendal's mother when he refused any weaponry at all to fight Grendal. He knows his limits.
In the film version, however, we see a more modernized hero. Though that version of Beowulf is heroic, I would argue that he is less heroic. He doesn't live by the same standards as the text version of the same character. In the film, not only does he fall to Grendal's mother's temptation, but he lies about it and he also has a mistress. He is brave, but he is not exceptionally noble. The original Beowulf is a larger than life epic hero, whereas the film version is a modernized, guilt-ridden man who completes some physically heroic feats.