Do you think that Grendel is Beowulf's doppelganger (or "twisted double")?

What an interesting idea to consider.
Beowulf is an epic Anglo-Saxon poem/story which chronicles the
exploits of Beowulf. One of the three adversaries whom Beowulf battles and defeats is
Grendel. Because they are opposite in nearly every way, perhaps Grendel can best be seen
as Beowulf's mirror image rather than his doppelganger.

We
do not learn very much about the monster Grendel, we are told that he "nursed a hard
grudge" (Seamus Heaney translation). This bitterness causes him to strike out in anger
and evilness throughout Hrothgar's kingdom. He goes on a killing rampage. We never hear
him speak or see him interact in any way with society. He prefers the dark and the
dank.

Beowulf, on the other hand, has nothing but noble
motives for his actions. He comes to Hrothgar's rescue with the lofty goal of killing
the marauder. He makes lofty speeches and enjoys the company of his fellow soldiers and
Hrothgar's court. He does not have any obligation to fight for Hrothgar, but he is a man
of honor and justice.

Beowulf has heard that Grendel fights
without weapons, so he plans to do the same. The battle becomes a literal arm-wrestling
match, each of them with an extraordinary strength in their arms (hands). The visual of
the two men locked in a grip of death is suggestive of a mirror image. Though the image
ends as Beowulf is the victor and Grendel slithers away to his marshy fen to die, the
two are clearly opposites--thus the mirror image.