One of the most important things to note about Beowulf is that he doesn't fight men at any point in the poem. The wars and clashes that happen between humans don't directly involve him. He sometimes argues with humans, like he does with Unferth before the battle with Grendel, but Beowulf fights monsters. This shows him as a defender of humanity and truly on the side of good.
Most people in Beowulf aren't portrayed as purely good or evil.