How are Grendel and Beowulf similar characters and how are they different?
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.
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Besides both being ferocious fighters, Grendel and Beowulf are very different. Grendel is described as a descendant of Cain, the first murderer in the Bible. This would make him evil in the eyes of an audience of his day. He lives in the wilds, away from human society, and his attack on Herot is motivated by his hatred for joyous celebration. On the other hand, Beowulf is a man who has deep affection for his other humans. He is not merely an incredibly strong fighter but he is equally skilled with words. He defends Unferth, talks like an ambassador when returning home, and, towards the end of his life, gives an accounting of his actions that sounds like an elder statesman. He is loyal, compassionate, and the only character who talks about God. Grendel seems capable only of destruction. Beowulf becomes a wise and successful king who is loved by his countrymen.