Why does the poet choose Cain to be the forefather of Grendel?

As far as I know, no definitive answer has ever been given to that question. The answer must be inferred from what we know about Cain, son of Adam, and brother of Abel. Cain killed his brother, and thus, was banished from his home. His time in exile made him a wanderer.... a fugitive, never accepted, shunned from society. Grendel is called a son of Cain, thus, shunned and cursed to be a wanderer.... to never belong.

Grendel

What about Grendel's action, motivation and relationships make him similar to cain?

What Does the conflict Between the dones and grendel symbolize?