Beowulf is only a king in the final part of the poem, but he is always a hero and eventually dies a hero’s death from the wound he received from the dragon in battle. It a standard convention of epics that the heroes should be of royal birth. For example, Gilgamesh and Odysseus are kings, and Achilles (since Peleus is still alive at the time of the Iliad) is a prince. Yet we do not think of any of these men primarily as kings, since their fame comes from the adventures and exploits in which they participate as heroes. Gilgamesh does much more harm than good when he stays at home in Uruk and his counsellors beg him to go out on quests. The Odyssey has a rather uneasy, unsatisfactory ending (which is the inspiration for Tennyson’s poem "Ulysses"). The opening lines make it clear why the story of a king is so much less satisfactory than that of a hero.

It little profits that an idle king,
By this still hearth, among these barren crags,
Match'd with an aged wife, I mete and dole
Unequal laws unto a savage race,
That hoard, and sleep, and feed, and know not me.

The story of Beowulf as king is not told because it would be too much like this. The reign of a good king makes a tedious story; it is only crisis that provides drama. Telling the story of Beowulf as king, however, would necessitate the choice between telling a dull tale or making him out to be a bad king—or at least an unsuccessful one whose reign is perpetually in crisis. This is why fifty years of peace are rapidly passed over in favor of the hero’s dramatic exploits.

Beowulf is an epic, and its focus is on narrating the heroic events of the protagonist. Thus, by only addressing Beowulf as a king, the poem would not fully address his heroic deeds prior his ascension to the royal position and conform to the fundamentals of the poem.

Beowulf is depicted as a man of valor and supernatural strength. In his youth, he fights and slays different monsters. For instance, during his swimming competition with Breca, Beowulf comes second because he is forced to fight off sea monsters. He actively seeks glory, and his pursuits lead him to the Danes as a savior. 

He goes to the aid of King Hrothgar after Grendel unleashes terror on his people. He fatally wounds Grendel and manages to kill Grendel’s mother when she tries to avenge her son’s death. Most of Beowulf’s heroic deeds occur before he becomes king. Thus, Beowulf is not only depicted as a king but an ancient hero with his glorious deeds revered in folklore.

The difference between the two is that his story as a king would have focused on his deeds as a royal leader, but the story covers more than his kingship and is focused more on his heroic deeds.