it is important to Grendel that his death be viewed as an accident as it will give honor to the king and not Beowulf.
He tells Beowulf that he is not victorious, because he just slipped on the blood and he wasn't actively pursued by him. On top of that, when he is running out in the woods he shouts that Beowulf's victory was an accident. Grendel strongly believe that Beowulf winning over it is completely accidental. Besides, if Grendel wouldn't have slipped, Beowulf would have lost.