Well, because _Beowulf_ is a Christianized retelling of earlier more pagan versions adding in moralizing (and of course involves a fair bit of nationalism/ethnicism although most people tend to ignore the allusions to the backstories and dynasties because it's hard to keep straight even if it's equally hard to miss that Beowulf dying is considered very ominous for the survivors), so it follows the same pattern as King Arthur discussed in OP: what was originally a relatively neutral set of stories about a warrior fighting monsters then becomes morally inflected - Grendel is not just a bandit or some monster, but he's a descendant of Cain, the first murderer and fratricide, inimitably evil and opposed to all mankind and core Anglo-Saxon moral values like hospitality and feasting which he cannibalistically perverts, and his dam can only be slain by a weapon from Genesis.