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?
Grendel has been alive long enough to have witnessed the creation of Hrothgar's kingdom, and he knows the truth of the past. When the Shaper comes, Grendel is as entranced as the men by his songs, but he also feels uneasy, because he knows what he sings isn't true. So Grendel believes Hrothgar's dream is doomed because of human nature. They aren't on the side of gods, as they imagine: they are as lost and purposeless as anyone else.
Grendel also takes issue with Hrothgar's idealized view of his sons. Grendel says that like himself, no one listens to Hrothgar, least of all his sons. Instead, they have their eyes on his wealth and his kingdom. Since Hrothgar's dream is to give the treasure away to everyone, Grendel says this idea is doomed. Hrothgar's sons won't be willing to give everything away; they will keep it all for themselves.
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.