It's always tricky to think through the motivation of the villain (especially when the villain is an inhuman monster), The original poem introduces Grendel thus (using Seamus Heaney's not-quite-literal translation):
|Ðá se ellengaést||earfoðlíce||Then a powerful demon,||a prowler through the dark,|
|þráge geþolode||sé þe in þýstrum bád||nursed a hard grievance.||It harrowed him|
|þæt hé dógora gehwám||dréam gehýrde||to hear the din of||the loud banquet|
|hlúdne in healle||þaér wæs hearpan swég||every day in the hall,||the harp being struck|
|swutol sang scopes||and the clear song||of a skilled poet|
The original Grendel is evil by nature, but is driven to homicide by sounds of revelry. The 2007 Robert Zemeckis animated version actually suck fairly closely to this motivation, though there is the extra wrinkle that Hrothgar turns out to be Grendel's father; the 2005 Beowulf and Grendel, starring Gerard Butler, has Hrothgar killing the young Grendel's father and thus providing a motive. John Gardner reimagines Grendel as a philosophical monster both attracted and repelled by the world of humans; he reminded me very much of Meursault in L’Étranger, except that I think he would have made a more entertaining dinner companion (as long as you weren't on the menu yourself).
I didn't spot the astrological references at the start of each chapter until tipped off by Wikipedia. Indeed, in general I felt this novel was too clever for me, but it is mercifully short with some passages done in interesting styles; and Grendel himself comes off as a believable character.