What is the difference between a good warrior and a good king in Beowulf?
In the time in which Beowulf was first told (it had been handed down in the oral tradition long before it was ever written down), survival was something the men and women of the day dealt with constantly: it was as common as taking a breath.
Those individuals who survived would have been strong warriors; and in the Anglo-Saxon world, this applied to men and women who fought side-by-side. Being a great warrior, however, spoke to individual survival, not necessarily to that of a group. A community could only survive with the collective skills of its warriors. And as great warriors were valued among the members of this civilization, it was the strongest and wisest warrior who would be king.
True enough, there would be leaders who ruled only by strength and force of will, but in Beowulf, the king was a man who commanded respect because of his wisdom, courage and integrity. These were the characteristics that would command the allegiance of the strongest of warriors. Many men could be excellent warriors, but not every warrior could be a king.
Beowulf had served such a king during the time he was proving himself on the battlefield--showing himself to be a man of great valor. Marital experience was only one aspect of Beowulf's character, but coupled with his just and fair-minded treatment of others, he won the respect and regard of his society, and he ultimately became the king of his people.
Being the strongest warrior held an important place within this coarse and primitive culture, but the qualities of a leader went well beyond strength, and looked to the character of the man and his ability to guide his people in the best and worst of times.
The difference is that the king has additional duties (politics). The king has to posses virtues of a warrior (bravery, honesty, loyalty) but he also has to award the warriors with gifts (gold, rings, weapons).
The warrior must be loyal to his king and fight bravely and thus respect the comitatus code. The king has to be just and protect his people by gathering warriors to fight in his name. Beowulf is an example of both. He was first a warrior and then a king. Furthermore, he was extremely successful in both roles.
Moreover, Beowulf confronts the dragon in his late years which is not neccessary a duty of a king. All warriors( except one) run away and Beowulf is both a warrior and a king when he fights the dragon in order to protect his people.