When the Beowulf poet describes his hero fighting evil, it is important to understand that the poem expresses a specifically medieval Christian conception of evil. Although scholars have debated and argued over whether these Christian passages which justify the fighting through defining the poem's monstrous antagonists as "evil," the passages as they exist in the text of Beowulf seem like straightforward moral glosses upon the action which occurs in the poem. By the time Hrothgar invokes for the final time the recurrent Christian definition of evil that runs throughout the poem, it is clear that each definition of evil is artfully arranged by the poet or compositor. If the scholarship is correct which suggests the Christian lines in the poem are, in fact, a later insertion, I will argue that the coherence of these passages, and the inability to remove them from the text as it exists, make them the effective moral of the poem, whose imagery of evil I think may contain some darker elements than might otherwise be expected.
The question of how Christian a poem Beowulf is, precisely, has been a vexed one among scholars of Anglo-Saxon heroic verse for a long time. The difficulty, of course, lies in the subject matter of the poem, which seems more like an epic tale of heroic deeds. It is worth considering what Kohler (one of the first to assess the Christian passages in the poem negatively) says about Beowulf:
Not only are there single Christianising passages, more or less clumsily inserted, which allow us to recognize unmistakably the clerical reworker, but a Christian colouring has been given to the entire poem, out of which the old heathen background often appears, so that here and there totally different outlooks come into close proximity. Just because of this thoroughgoing and carefully carried-out Christianisation of material which belongs to old native legend, it is not possible to content oneself with cutting out single lines and some of the larger passages in order to gain the poem's old text. If one leaves out all the lines and passages indicated by Ettmuller as later Christian additions, there still remains enough that is Christian to recognize clearly the Christian reworker, even after one has cut out all that most conspicuously stands in the sharpest contradiction to the heathen material. (Shippey and Haarder, 355)
This betrays a much more contemporary suspicion of the poem precisely for its religious content, rather than investigating more closely how such controversial content ended up in this format.
In any case, the first of the "Christianising passages" in Beowulf is presented very early in the poem. The passage itself runs from lines 99-114 in the Heaney translation, and it represents the initial introduction of Grendel at the outset:
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. For the killing of Abel
The Eternal Lord had exacted a price:
Cain got no good from committing that murder
Because the Almighty made him anathema
And out of the curse of his exile there sprang
Ogres and elves and evil phantoms
And the giants too who strove with God
Time and again until He gave them their reward. (Heaney 9)
In other words, the poet in Beowulf traces Grendel's genealogy to literally an antediluvian start-date. Grendel is likened to Cain, who was damned by God and made a "wanderer on the earth" for the killing of Abel. In an important essay on this genealogy presented for Grendel, Mellinkoff is careful that we do not mistake Grendel for anything more grandiose in its wickedness than the aboriginal human malice of Cain -- in other words, Grendel is not supernatural or a demon; as Mellinkoff puts it,
While they incorporate a spirit of evil, Grendel and his mother are not primarily evil spirits. Hence their identification as evil or demon spirits by some interpreters falls short of the mark. They are not demons or devils in the unsubstantial immaterial sense, but rather are tangible, solid creatures. They die natural deaths and are sent to hell to await judgment. Of course they are devil-like, but not transparent or shape-changing beings; as Tolkein expressed it: "Grendel is not a real mediaeval devil except in so far as mediaeval bodies themselves had failed (as was often the case) to become real devils." (Mellinkoff 151)
The distinction Mellinkoff makes is important. We need to recall that the tremendous age of the Biblical genealogy offered to Grendel by the poet does not indicate anything but the antiquity of this race of "giants…who strove with God," which identifies them with the giants presented in Genesis after Adam and Eve's expulsion from the garden.
Kohler's sense of the "interpolated" quality of the Christian passages continues, because the next time the issues of Grendel's evil surface it is to consider Grendel's mother. Here is where the origin dating to a wickedness before Noah's flood becomes important, because of course Grendel's mother lives underwater, which is apparently because she was forced there by Noah's flood (intended to cleanse human wicknedness "from the face of the earth," as presented in Genesis). But the first thing to note about the next description is its similarity to the first occurrence -- we are being deliberately reminded of the initial passage referring to Grendel, as the poet takes more time lingeringly applying the same logic to Grendel's mother in lines 1258-1274 of the Heaney translation:
Monstrous hell-bride, brooded on her wrongs.
She had been forced down into fearful waters,
The cold depths, after Cain had killed
His father's son, felled his own
Brother with a sword. Branded an outlaw,
Marked by having murdered, he moved into the wilds,
Shunned company and joy. And from Cain there sprang
Misbegotten spirits, among them Grendel,
The banished and accursed, due to come to grips
With that watcher in Heorot waiting to do battle.
The monster wrenched and wrestled with him
But Beowulf was mindful of his mighty strangth,
The wondrous gifts God had showered on him:
He relied for help on the Lord of All,
On His care and favour,. So he overcame the foe,
Brought down the hell-brute. (Heaney 89)
The "branded an outlaw" at line 1263 is also specifically a reference to the Biblical "brand of Cain," the mark placed on his forehead by God as a sign of his guilt. Orchard notes that the Anglo-Saxon word for "branded" used here is also taken from the Bible's own cain narrative -- Orchard notes that "in Beowulf we are told that Cain went 'guilty' or 'marked' (fag) into the wastes….its similar phraseology is clearly intended to recall the first allusion to the kind of Cain" (Orchard 60).
In conclusion it is worth looking at the final, memorializing, reference in the poem to the genealogy of the evil that Beowulf must fight in Grendel and his mother. This comes later, at lines 1687-1693 in the Heaney translation, when Hrothgar (at lines examines the sword which bears precisely such an illustration -- in other words, the sword (or emblem of warfare) seems to have been designed (in "old times") precisely for the purpose to which Beowulf has put it: