The most obvious superficial difference between Beowulf and Superman is that Beowulf is mortal, Superman is not. Superman is destructible but he is not human and he is invulnerable to earthly threats. "Beowulf" purports to describe historic, albeit fantastic, events, while the Superman stories narrate fictional and decisively superhuman events. Beowulf is the consummate warrior defending his people, whereas Superman is a superhuman force defending human ideals.
Beowulf is a member of the community he defends and his deeds are widely published; the poem is written to immortalize him. Superman is an exile from the destroyed planet Krypton who conceals his identity. The heroic ideal in Beowulf's time was not remote and the hero was active in community life. Superman plays a community role but not in his own identity or his own community. Beowulf's heroism derives from his fame, while Superman's heroic feats depend on his anonymity.
A hero is defined by his culture, and key differences between Beowulf and Superman derive from the time they lived. Beowulf and Superman each embody the Aristotelian concept that courage is "the first of human qualities," and each represents the ideal man of his era defending the moral justice of his community, but the morality and the communities are distinct. Beowulf embodies the heroic ideal of kingly behavior of pre-Christian Anglo-Saxon society; Superman is a visitor whose strength compensates for humans' inability to defend themselves.
Beowulf dies at the end of the poem and it is essential that he die as death is the source of his heroism and his immortality. Superman is impervious to earthly threats and it is essential to Superman's heroism that he not die. Beowulf's definition as a superhero in his time depends on his epitomizing humanity, whereas Superman's superheroism depends on his surpassing humanity and the implication of frailty, including death.