For Spencer's benefit, I've illustrated my post with a picture of Grendel. Somehow I feel that this depiction isn't too accurate though.
Grendel sees Beowulf as being insane - in some ways it seems that Grendel could even see Beowulf as not being human (for example when Grendel thinks that Beowulf suddenly has wings) although on pg. 169 Grendel refers to him as his "dear long-lost brother, kinsman-thane" and says "he is only a man."
In Beowulf, it says that Grendel was killed by Beowulf's extreme strength. However in Grendel Beowulf seems to win through his rhetorical tricks and brainwashing.
Grendel refuses to accept that Beowulf could possibly be stronger or smarter than he is. He continually reminds himself that Beowulf caught hold of him through trickery and accident. His last words echo this: "Poor Grendel's had an accident." He cannot believe that any living creature could get the better of him. He thought that he was immortal and this new realization is hard for him to believe.