Well, In the beginning His response is not much different, As Grendel only sees Beowulf as an Average citizen. Grendel is not fearful when they first meet, in fact (if i remember correctly) he is excited that Beowulf and his followers had come (So he could have more victims)
It's not until Beowulf Grabs Grendel "By the claws" That he's fearful. once beowulf shows that he's just as strong/maybe stronger Grendel becomes scared and "Wishes he could run away"
Normal Citizens were easy for Grendel to target, and easy victims for Grendel to Slaughter for his enjoyment. Beowulf is was not.
