Antagonist
Character Role Analysis
Hrothgar. Unferth. Dragon. Beowulf. Pretty Much Everybody.
At some point, just about everyone in this cast of characters plays the antagonist to Grendel's protagonist. We expect that Beowulf would be the natural antagonist of this story—and he is. He comes with the intention of destroying our main character, and there is definitely something creepy and unnatural about this guy.
But if we think like Gardner for a second, we can't deny that several other characters are good candidates for the role of antagonist, as well. Hrothgar, for one, does a lot of stupid and nasty things: he bullies lesser kings, snatches away a beautiful young woman from her hearth and home, indiscriminately consumes natural resources, and kills people without eating them. Okay, we're on the fence about whether that last one is bad or good or something else. In any case, Grendel certainly wants to wage war on him for these offenses.
Unferth is a slobberer and generally a disgusting human being; he violates rules that Grendel would never break, and he's a total poser. And then there's the dragon. He destroys any shred of hope that miserable Grendel ever had about his purpose in life—and that feeling doesn't just end when their meeting ends: it follows Grendel and drags him down till the end of his days.
So the antagonist? Pick one. Pick many. They're all over the place.