We're told that Grendel 'nursed a hard greivance,' though its nature is somewhat vague -- it seems to have to do with a sense the that building of Heorot and the sounds of feasts within offended Grendel, as it was previously part of his domain (though one might point out, that from the point of view of monstrous appetites, it was rather like someone building a McDonald's next door to one's house). But unlike Grendel, Grendel's mother has a very specific grievance that the Saxons and we today can all instantly recognize: revenge for the death of her son.
But there are other, perhaps less-obvious points of comparison: Grendel's mother lacks a name (though in a poetic form averse to proper names, it may not be a handicap), and her visit to Heorot is quite differently framed; despite its ferocity it is fringed with fear: "the hell-dam was in panic; desperate to get out / in mortal terror the moment she was found." She had come, it seems, not so much for revenge as to retrieve her son's arm, which had been hung as a humiliating war-trophy from the cross-beam.
Hrothgar, belatedly, tells that he has heard tales of "two such creatures," and gives Beowulf directions to the mere (!) -- one wonders why he didn't say something about it sooner. And, though the response of the hero is no less bold than before, the battle is quite a different one: it takes place unseen by comrades, and Beowulf comes disturbingly close to defeat. His eventual victory comes by using like against like; only a weapon from the age of giants can slay a giant. His faithful comrades go home, believing he has lost, and he has to go after them to announce his victory, perhaps a foreshadowing of the faithlessness his retainers will show at his final battle with the dragon. All in all, it's a bleaker, lonelier episode, one that all the rich rewards offered at the second feast, it seems, can scarcely recompense for. Even for the boast-loving Saxons, there is a sense of hubris.