What aspect of Beowulf's battle with Grendel tells you that Beowulf is honorable?

Beowulf is an ancient poem which
tells the story of an epic hero, Beowulf.  In the course of the story, Beowulf engages
in battles to the death with Grendel, Grendel's infuriated mother, and a gigantic
dragon.  The battle you mention in your question is the first of these three battles,
and it happens shortly after we meet Beowulf for the first time.  He and his men have
made a dangerous sea crossing in order to help save Hrothgar and his people from the
marauding monster named Grendel.  Beowulf has learned that Grendel never does battle
with weapons, preferring to do any fighting in hand-to-hand combat.  Beowulf does an
honorable thing by deciding he will battle Grendel on the creature's own terms--he will
not use a weapon when they battle.  He doesn't, of course, and Grendel loses in a fair,
hand-to-hand fight (literally).  This, among other things, demonstrates Beowulf's
honor.