Today we may look at Fantasy Literature as just another genre of literature. And though the world was full of the tales of myth at the time that Beowulf was written, Fantasy Literature was virtually non-existent. So why would a monk document a story that involved such an inconceivable creature as Grendel? There are many possible answers to this question (many of them viable), but I would like to focus on the possibility that Grendel was actually a historical figure. That he was some sort of early serial killer that the monks of that time period (or those telling the story previously) needed an explanation for. It is plausible that the horror of someone who would slaughter people needlessly called for rationalization even if it defied reason. Indeed it is only logical that people would search for a motive that "this mead-hall was seen all stained with blood: / blood had soaked its shining floor, / it was a house of slaughter" (lines 485-487).
Even today as we watch the news and hear of people who go out and kill repeatedly we are sickened that they could be a part of the human race. It is astonishing that such horror could exist in a fellow human being. Today, however, we have the benefit of psychological data that can help account for a person's viciousness (though not necessarily excuse it). They may have a chemical imbalance; they may have a mental illness or have a history that led them to kill. And there are still those whom we may see as just purely evil. But we may imagine that if such a man existed in the historical context of Beowulf that his presence would beg for some explanation.
If we look at Grendel from a more modern perspective we might see him as more complex that just a mindless monster. Perhaps he reasoned that he was doing a noble thing by instilling fear in Hrothgar and his warring plunderers. Perhaps he was merely reacting to his personal feelings of loneliness and exile. Perhaps if Grendel's character were more fully investigated we would find that he is a man-and was then-who is misunderstood; and that the result of this misunderstanding is anger and hatred that are manifested in a most brutal manner.
In Grendel's historical context there are avenues he can take where personal vendettas and grudges are concerned. But Grendel was apparently not guided by any of his society's collective laws. The Beowulf author describes Grendel's complete disregard for the traditions of that time period in lines 150-162:
…Grendel warred
long on Hrothgar, the harms he did him
through wretched years of wrong, outrage
and persecution. Peace was not in his mind
towards any companion of the court of Hrothgar,
the feud was not abated, the blood-price was unpaid.
Nor did any counselor have cause to look for
A bright man-price at the murderer's hand:
The dark death-shadow drove always against them,
Old and young; abominable
He watched and waited for them, walked nightlong
The misty moorland. Men know not
Where hell's familiars fleet on their errands!
This passage illustrates that Grendel, having no concept of the rules governing "blood-feuds," or choosing to ignore them, must have been on "hell's…errands." If he perhaps had some wrongdoing to revenge he does not go through the established channels to avenge them. Because of this barefaced disregard for the mores of the community, his behavior must be associated with that of hell.
These facts also explain one reason for the references to Christianity in the Beowulf text: if there exists a malicious being who makes a habit of killing people without reason, he must have descended from Cain-the father of all murder. Therein lies a somewhat reasonable account for a Christian audience to explain how Grendel-or someone like him-could possibly commit such atrocities even though the actual characters in the tale are not Christian. The author expounds on this idea of Hrothgar's pagan society not understanding the nature of Grendel. One example is when he describes how Hrothgar's people would pray in lines 174-179:
They prayed aloud, promising sometimes
On the altars of their idols unholy sacrifices
If the Slayer of souls would send relief
To the suffering people.
Such was their practice,
A heathen hope, Hell possessed
Their hearts and minds: the Maker was unknown to them
And because they had no conception of hell and its demons, they would not realize, as the Christian audience would, that Grendel could be explained as one of these devilish beings.
Obviously the character of Grendel-whether patterned after an historical serial killer or not-would leave people searching for some kind of explanation. It is horrifying (even today) to realize that such people even exist. And when, at a time when any possible explanation for such horror could not be found, perhaps the most logical thing to do (perhaps unknowingly) was to create a new genre of literature where a being of Grendel's kind isn't even human. And as a result of this new genre-or Fantasy-there is a way to deny that people like Grendel exist at all.