Why does Grendel believe Hrothgar’s dream (that he could one day pass his kingdom on to his sons and his sons’ sons) is doomed in Grendel?

In Gardner's Grendel, Grendel knows, or at least strongly suspects, that Hrothgar's children will never rule his kingdom.  He knows that Hrothulf will kill Hrothgar's children as soon as Hrothgar dies.

Grendel writes that Hrothulf, at age fourteen and recently arrived at Hart due to his father's death, is already a "pretender":  a pretender to the throne.  Grendel writes that Hrothulf sits between Hrothgar's children and sharpens his knife.  Grendel tells of Hrothulf's conversations with his tutor, in which Hrothulf displays Machiavellian tendencies to power. 

Grendel has seen enough of humans to know that Hrothulf will eliminate Hrothgar's children, since they are in line to inherit the fiefdom before him.  Son of Hrothgar's younger brother, Halga the Good, Hrothulf will seize power the first chance he gets.