Grendel and his mother are descendents of the biblical Cain. Grendel was the reason Beowulf left Geatland, becaise Grendel had been attacking the mead-hall at Heorot, killing and eating anyone he found there. Grendel's reason for attacking were justified in these times, as the noise from the drunken revellers at Heorot's mead-hall was disturbing him.