This is an attempt to tell the story of Beowulf from the side of the monster, an experimental early 70s novel (with super illustrations by Emil Antonucci), which obviously ends with Grendel's death, though his mother and the dragon are there in the background.

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):

The original Grendel is evil by nature, but is driven to homicide by sounds of revelry. The 2007 Robert Zemeckis animated version actually stuck 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.