Identify one hero and one villain from Beowulf.

The main hero of Beowulf is Beowulf, and a central villain is Grendel. I’ll start with Grendel.  Grendel is definitely the "bad guy" and not simply because the hero fights against Grendel.  Grendel is the villain because he’s flat out evil.  It’s not clear if Grendel is a man or a monster; however, it doesn’t matter.  Grendel is evil incarnate.  Readers are told that he is a man-eating demon from hell that has descended from the biblical line of Cain.  Lines 99–107 tell readers the following information about Grendel.

So times were pleasant for the people there until finally one, a fiend out of hell, began to work his evil in the world. Grendel was the name of this grim demon haunting the marches, marauding round the heath and the desolate fens; he had dwelt for a time in misery among the banished monsters, Cain's clan, whom the creator had outlawed and condemned as outcasts.

Grendel is monstrous and enjoys violence for no other reason than that he finds pleasure in doing violence against people.  He likes killing, and he’s good at it.  That’s why no warriors have been able to defeat him.  
Beowulf is the poem’s hero, and he is a standard Anglo-Saxon hero.  In fact, that hero type is still alive and well in heroes from modern culture like Captain America, Wolverine, and Batman.  First of all, Beowulf looks like a stereotypical hero: he is a muscular “manly man.”  This helps Beowulf look like a warrior.  He’s intimidating just to look at.  This is confirmed to readers early in the poem when Beowulf is on his way to see Hrothgar.  A guard stops Beowulf and demands to know Beowulf’s purpose.  Beowulf is such an amazingly awesome warrior to look at that the guard even comments on Beowulf’s heroic appearance.  

"Nor have I seen
A mightier man-at-arms on this earth
Than the one standing here: unless I am mistaken,
He is truly noble.  This is no mere
Hanger-on in a hero’s armour."

Regarding Beowulf’s musculature, I believe that having the strength of thirty men counts as really muscular.  

"Who valuable gift-gems of the Geatmen carried
As peace-offering thither, that he thirty men’s grapple
Has in his hand, the hero-in-battle."

Probably my favorite piece of evidence of Beowulf’s strength is the fact that he rips Grendel’s arms off with his bare hands.
Beowulf is also a good example of a hero because he is brave.  Remember, Beowulf chooses to fight Grendel. Grendel has proven to be unbeatable, and very brave men have died fighting Grendel.  Despite that fact, Beowulf doesn’t hesitate to challenge Grendel.  Beowulf doesn’t fight only when he’s backed into a corner.  Beowulf is the type of man that willingly runs toward the danger.  
Lastly, Beowulf exhibits the standard heroic quality of being humble.  He knows that he is good at what he does, but he isn’t cocky and arrogant.  He is a powerful opponent in battle, and he commands great respect and power from his peers, yet he doesn’t ever abuse that power.  A good example of this is when Beowulf is offered the Danish throne and the immense wealth that comes with it. Basically, he’s offered fame, fortune, and more power, but Beowulf turns it down and returns home the same way that he left.