At first Grendel meant Hrothgar’s soldiers no harm. Why, then, did they attack him?

When Grendel first meets Hrothgar's men, he is overcome with curiosity, as are they. If fact, the men believe Grendel to be a tree spirit and attempt to feed him. It isn't until Grendel tries to verbally communicate with the men that they become frightened and attack him. They are unnerved by Grendel's howls.