Why is Hrothgar left unharmed?

A very good question! Many are killed in the early days of Grendel's attacks, but presumably Hrothgar simply stopped using the hall for feasts and kept his men away. He certainly has followers ready to do his bidding; he maintains his coast guard who welcomes Beowulf to Denmark and also a door-warden who challenges the party as to their business. Later we hear of a troop of young men who go riding out along the trail and Grendel's mother kills Aeschere, an older earl of the company.

It is a good question because there is the issue of what is denied to Hrothgar by Grendel: his hall and crucially his gift-stool. The best warriors were drawn to liberal lords eager to share wealth. If Hrothgar could not attract such men, his following may have been less than impressive. The character of Unferth for example seems to have esteem in Hrothgar's hall, yet it would appear that he was a fratricide; not a man that most lords would entertain. Perhaps Hrothgar was not in a position to be choosy.