Beowulf is the hero and Grendel the anti-hero. If you set up a chart and create a list of heroic ideals, you'll see that Beowulf and Grendel are polar extremes.
Valor: Valor is courage in defense of a cause, generally a noble or elevated cause. Beowulf shows tremendous courage in traveling to a land that is not his own in order to fight a monster attacking the joyful. Grendel, on the other hand, bullies his way into Heorot with terror on his agenda, but when faced with a true battle, he runs away, leaving his arm behind.
Selflessness: Beowulf certainly hoped for glory, but he could have obtained that fighting military battles. His motive was to rid the Danes of the monster. Grendel's motive was to quell the celebrations and darken the happy halls because he found them offensive.
Loyalty: Beowulf's loyalties lay with his king and with Hrothgar. His treasures were passed among the soldiers who accompanied him, and he refused to become king of the Geats until all the other heirs to the throne had served and died. Grendel was loyal to no one but himself.
And the list goes on...