What is the significance of Grendel being descended from Cain?

Grendel is descended from Cain because they are both evil. Cain, brother of Able was the first "evil" person. He killed his brother which led God to banish him, just as Grendel was banished by God. Cain was banished and decided to "build up" his forces such as ogres, elves, phantoms, and giants. Grendel is kind of a giant and ogre combined. In conclusion, Cain is wicked freakin' evil and they wanted to show a connection between him and Grendel to show just how evil Grandel was.