What explanation is given for why the menace of Grendel has stricken the Danes?

Grendel has a lot of emotional issues. He is kin of Cain and hence God has cursed him. Grendel can't stand listening to the merriment in Heorot Hall. He spends most of his time at home and he lives in a swamp with his mother. Grendel especially hates listening to the praises of a Christian God who has cursed him to a miserable existence. Like most angry monsters, he lashes out at everyone that makes him feel inferior and sad.