The Beowulf poet is quite clear about the origin of Grendel's hatred of Hrothgar's men and, later, Beowulf and his men.  As the descendant of Cain, Grendel has been cast out of all human fellowship:

. . .  [God] had condemned him as Cain's kinsman.  The Captain Eternal/. . . avenged the murder of Abel by Cain/and banished him [Grendel]. . . away from humankind. . . . (ll. 106-110)

At any time that Grendel realizes that men are celebrating in Hrothgar's mead hall—a celebration which he cannot share because he is a descendant of Cain and therefore an outcast—Grendel rebels against his exiled state by fighting against God and his loss of man's fellowship in the only way he can—by attacking men.

The notion that Grendel's hatred of men is unnatural is made clear when the Beowulf poet recounts Grendel's merciless attacks on Heorot:

. . . Grendel fought a long time against Hrothgar/ hate-feuds he waged,/crimes and murders, for many seasons . . . he wanted  no truce/with any man of the Danish forces,/ to let them pay for peace. . . . (ll. 151-156)

In Anglo-Saxon and Scandinavian societies, warfare, which was common in these societies, was marked by certain conventions—the chief of which was the payment of money for peace.  If a murder were to occur between two tribes, for example, it was common for the offending tribe to pay wergild—that is, a man price—to compensate for the murder.  On a larger scale, disputes between tribes over land were often settled the same way.  The fact that Grendel will not accept payment places him outside the normal conventions of men's behavior and is a sign to Hrothgar and his people that Grendel is an unnatural being, a fact made clear by his unceasing and unprovoked attacks on Heorot.

In sum, then, Grendel's hatred for Hrothgar's men—all men, for that matter—is derived from his outcast state; his antipathy stems from his complete isolation from mankind's joys and fellowship.