Grendel only ever hunting humans at night and when they were sleeping. Warriors were getting really scared and in order to escape him, they went far away just so they could get some sleep. Grendel only attacked the men because he couldn't attack the king. The king's throne was protected by God.
Sailors have come from Danish telling the tale of Grendel and the horrible things the monster have done to the villagers. The King of the Geats told Beowulf that he was the only one who could defeat the monster. So Beowulf gathered 12 strong men and went on his way.