The protagonist and antagonist of a story are determined by perspective, actions, and state of mind. I believe Grendel is both, throughout the story he constantly changes perspectives and takes new action thus making him a dual sided character. In the beginning he has a thirst for curiosity, almost like a child. As he matures and interacts with different characters he begins to find his true purpose and decides for himself what he has to do.
In observer does not hurt anyone and Grendel was just curious as to how humans behave. Being the protagonist at this point, he observes the disgusting ill mannered behavior that humans label normal and right. He is a child being influenced in a negative way and this foreshadows his change. If Grendel was truly innocent in the beginning of the novel, which he was, then his perspective was obviously changed after encountering certain things. The dragon enlightened Grendel as to what his longed for purpose was. It said that Grendel needs is meant to wreak havoc on the Danes and torture Hrothgar until he dies. Grendel realized that he is enchanted with a spell that makes him invulnerable to weapons and that pushes him to finally confront the humans. At this point he is now the antagonist and enjoys killing and causing chaos in the mead-hall. His power and the way he is uses it reminds me of an evil villain trying to triumph over his arch-nemesis. Grendel’s innocence returns as he spares Unferth because once again his views are challenged. He cannot stop pondering why this man pursues him knowing he will die. After hearing why Unferth acts the way he does Grendel takes pity upon him and brings him back to Herot alive. This is symbolic to how a human can change his views in an instant if something is impactful enough.
Grendle’s constant shifts from protagonist to antagonist left him with a weak resolve. He set out to do certain things but failed because he did not follow through. He could not be just an observer because he ended up interacting with the humans and he failed to torture the Danes and Hrothgar because he allowed Beowulf to kill him. Grendel is a failure. He fails because he never found one goal to devote all of his energy to, but instead he constantly changed perspectives and acted on impulse.