The biggest obstacle to modern readers understanding the story of Beowulf seems to be the story’s attitude, and our attitude, toward monsters. What are monsters? Do they exist? Why are they monsters? What is their motivation to attack us? Are they irredeemably evil?
The modern answers to these questions: No, they do not exist; there are only animals and humans. They are “monsters” because we view them that way, we force them into that role for reasons of our own. Their motivations to attack are either animal (survival) or human (some kind of pain-based revenge or protection). They aren’t irredeemably evil, because that would imply that we are so, ourselves, at least some of us. In some extreme circumstance, the evil one can choose good.
The pre-modern answers: Yes, they do exist, and they are either the product of an unnatural coupling among animal, human and spirit, or they have some other unique, other-worldly origin. They are monsters because they just are, the way carnivores are carnivores, the way sea creatures breathe water. Their motivation to attack us is that it’s their nature to do so, because they really are irredeemably evil.
The only way to understand Beowulf’s story is to step into the ancient world and accept that monsters are monsters. They don’t have an understandable reason for being bad. They’re just bad. They may envy humans for being redeemable, but their response is to kill, not to try to reform.
Monsters came with the ancestral legends of the North Germanic tribes. In their mythology, there were gods and giants, with the giants representing destructive forces. Monsters were the offspring of giants, or perhaps of Loki, the one god whose alliance to good was questionable.
But newly-Christianized Anglo-Saxons had to find another way of thinking about monsters. The Bible pointed them back to the first wicked man, Cain, who did the worst thing the Germanic tribesmen could imagine: he killed his brother. Kin-slaying happened, but it was always a shock and often meant expelling the guilty party. So Cain was fingered as the progenitor of monsters. His lineage must have deliberately interbred with demons and monstrous animals.
But there’s one obvious problem with Cain as the progenitor of monsters: the Flood killed all humans except the family of Noah, who was not descended from Cain. One theory was that wicked men and monsters wrote down evil spells on granite, and after the flood they were discovered and evil revived.
So it really all goes back to Cain. And think about Cain’s story in the Bible: having killed his brother, he was made an outcast. To the Germanic tribes, being an outcast was the worst punishment. An “outlaw” meant someone deemed outside the protection of kinship law, someone who could be murdered without repercussions. An outlaw was afraid all the time, not sure who he could trust. An outlaw could try to settle somewhere and have a family, but his children would not be part of a kinship system, so they too would be outlaws. That’s one rational reason for monsters to hate humans: because humans have the joy of kinship. But while it may be understandable, it’s still evil to the core. There was no redeeming feature in any monster, in the Anglo-Saxon or Danish mind.