Beowulf was a hero in many ways. He was brave, kind, and willing to help others. The people of Denmark thought he was a hero in all of these ways, and more. They admired him so much, not only because he killed Grendel, but because he had all of these characteristics about him.
Kindness was one thing that made Beowulf a hero. He was kind to go to a land and fight of this monster for strangers that he didn't even know. He was kind to Hrothgar, even when the sword that he said would defeat Grendel's mother melted away into nothingness.
The people thought Beowulf was a hero because of his bravery. They thought he was brave for killing Grendel, and rejoiced after he did so. He was brave for coming to Heorot in the first place,