Let's face it: we know that something bad is coming, and we don't have to be fans of Beowulf to figure that out. Something bad is coming to Daneland, and Grendel can feel it in his bones and in the movement of the earth itself. He hears an old woman talking about a "hero" with unnatural abilities—and it excites him.
Grendel suffers from major fatigue by the end of his story: he can no longer stand the stupidity of existence—the constant changing of the seasons, the everlasting idiocy of human plans, the lack of purpose or sense of free will in his own existence.
So when Grendel finds that a new guest is coming to the party, he gets pretty worked up. And no matter what Mommy says, he's not going to miss out on the biggest thing to happen to Hart in twelve years. He can't possibly resist. So what if it might be his last battle? (He doesn't really believe that, anyway.) Remember all that stuff about I alone exist? Grendel takes that pretty seriously.
After all, Grendel's in a class by himself: he's got a dragon-charmed hide and monster strength, after all. When he gets into it with Beowulf, he doesn't realize how evenly matched they will be in strength, and he's totally unprepared for his opponent's nasty, sadistic streak. In Grendel's eyes, this battle is totally unfair. What's the point of being a cursed monster if you can't crush everything in your path?
And even though Beowulf is right there, larger than life, talking up his own version of the death match, Grendel tries hard to hold on to his narrative advantage. He talks about a new level of clarity that he's reached (his "sight clears" and his "sanity returns") in order to try to get control of the situation.
Even when he has to concede victory to that braggart freak of nature, he still tries to persuade us: "First you tricked me, and then I slipped. Accident" (171). It's all so unfair, right?
That's pretty much the reason for Grendel telling his story in the first place, isn't it? The monster's side always gets lost when the hero starts talking smack. That's why, as Grendel lies dying, he's got to get the last word in—and that last word has to even the playing field. If he's going to be isolated, cursed, tricked, and destroyed, he's going to take all of us with him.