A constitution is a statement of the basic principles and laws of a nation, state, or group, such as the U.S. Constitution. Another very common meaning of constitution is the physical makeup of a person. So think of a constitution as how a body (yours, the government's) is set up. If you have a strong constitution, it means you don't get sick very often.
I think the focus of the story is on Beowulf as a hero rather than a king because it is his heroic deeds that lead to him being offered the throne. Also to be king one must possess some type of heroic qualities in order to rule a land and its people.
A Comparison of Perfection in Beowulf and Sir Gawain and the Green Knight - Perfection in Beowulf and Sir Gawain and the Green Knight The heroes of both Beowulf and Sir Gawain and the Green Knight are fighters. However, the traits they have in common are far less numerous than those that set them apart.