Question

Why did the writers have grendel being hrothgars son

Answer 1

Well beowulf was an epic peom so there is no first hand source to tell you why. It was a story that was passed down from generation to generation. 

Answer 2

Explanation:  

I think this made for a more understandable plot point for people, as most of them were, who had never read the play before. Hrothgar would have had made a deal with the Devil and Grendel would have been the consequence. It's the sort of bargain that we might see in Faust or Macbeth.