The Spear-Danes in days gone by/ and the kings who ruled them had courage and greatness. / We have heard of those princes heroic campaigns. In the poem Beowulf, Beowulf is painted as a hero, a celebrated warrior, who is in search for 'heroic campaigns'. Grendel, on the other hand, is in search for the reason why he is a social outcast. Although Grendel and Beowulf play opposites in the novel and poem, some of their motives turn out to be similar to each other's; and even though there is great difference between man and monster, one can observe that in the end, in terms of motivation, Beowulf the man, is not very different from Grendel the monster.
In answer to the question as to what motivates Beowulf, the Heroic Code motivates Beowulf, and so does the thrill of different battles and challenges. Part of the reason that Beowulf comes to the rescue of the Danes is because he looks forward to the adventure that lies ahead; the encounter with Grendel. "I have come so far, o shelterer of warriors and your people's loved friend, that this one favor you should not refuse me- that I, alone and with the help of my men, may purge all evil from this hall." (Heaney 257-261). Battle after battle and challenge after challenge, he drives forward, his thirst quenched by the shedding of blood. As Jokinen states, "Beowulf does not expect to return from his fight with the dragon. Nonetheless, he enters the battle. It is such courage and loyalty to his people that will cause songs to be composed and sung about him" (Jokinen 48). This proves the point that Beowulf's motivation for action comes from his people praising him for his bravery.
On the other side of the sea, Grendel does what he does for several reasons. The main reason for this is his jealousy towards the 'regular, normal sized humans'. Even though in the beginning of the book Grendel, one will notice that Grendel does try to 'socialize' with the humans. As Gardner narrates Grendel's feelings, "It filled me with joy, though it was all crazy, and before I knew I could do it, I laughed" (Gardner 26).
Part of Grendel's motivation for terrorizing Herot results from his frustration over the fact that he is a descendant of Cain, and even though he thinks that this should be a good enough reason for him to mingle with the humans, it seems to him that the Danes don't share his sentiment. Not only that, but the sound of merriment makes him angry beyond reason. And he takes this anger out on Hrothgar's men, killing them in the most disgusting ways possible.
The other part of his motivation is the outcome of the jealousy that burns in Grendel's heart, making him aggressive to a point where he begins to show the true colors of a monster. As is stated in Beowulf, "It harrowed him / to hear the din of the loud banquet / every day in the hall, the harp being struck / and the clear song of a skilled poet / telling with mastery of man's beginnings ..." (Heaney 86-91).
The similarities in their motives come to light when they first encounter each other. In both the poem and the novel, Beowulf faces Grendel in a fight which results in Beowulf ripping off Grendel's arm. From this point forward, both Grendel's and Beowulf's motives move into a new direction. Beowulf seeks to finish the job of killing Grendel, while Grendel is determined to find out more about Beowulf's surprising strength. As Gardner states Grendel's thoughts, "I stare down amazed. He has torn off my arm at the shoulder!" (Gardner 172).
Another similarity in their motivation is that they both are offended. Grendel is offended at Hrothgar's every night celebrations, while Beowulf is offended by Grendel's ignorant and without-reason killings.
In her criticism, Jokinen states, "It is in battle that the mettle of the epic hero is tested" (Jokinen 22). Beowulf is labeled as a hero in both the poem and the novel. His courage is somewhat responsible for that, and this label in turn, motivates him to fight difficult battles and undertake challenges such as the one with Breca. "Truth I claim it, / that I had more of might in the sea / than any man else, more ocean-endurance" (Beowulf 533-535). And here is where the differences in both Grendel's and Beowulf's motivations lie. Beowulf seemingly has a justified reason to do what he does best, i.e. fight. This also proves that his killings have a rightful reason behind them. But in Grendel's case, one may not see the justification until he or she ponders over why Grendel kills. The reason would be that the normal humans don't want anything to do him just because of his appearance and his speech impairment. And it's not that Grendel did not try to be one of them; he just got rejected at face value. And although his murderous means might seem gruesome, social isolation is most of the reason why he kills.
Amidst all the differences between Beowulf and Grendel, the motivations behind their actions are much more similar than one would think them to be. And even though one is the hero and the other the antihero, One may criticize Grendel for his unjustified and violent nightly rampages, and that wouldn't be completely false; but if one were to observe in depth, one would see that Grendel is greatly similar to Beowulf. Nowadays, actions are justified just on face value. But as the proverb goes, 'Never judge a book by its cover'; it's a better idea to look beyond into what is the motive behind the action. Because sometimes, motives change perspective; just as it does in the case of Grendel and Beowulf.