What is the main theme of Beowulf, and is it sustained from start to finish?
If it can be stated as such, the main "theme" of Beowulf deals with honor and bravery, according to a code of ethics that was a precursor to the feudal code of medieval times. The values exemplified in Beowulf focus on the consensus of 'allegiance for protection' but more in terms of a lord's duty to his vassals than the other way around.
As Beowulf is the most ancient verse known in English literature, the story line, including of course character development, must be historically and culturally decoded for an appropriate appraisal. The same goes for the values and code of conduct embraced. As a teacher, I find the aspect of the heroic boast the most interesting element of the story.
As to the second part of your question, the fact that in the end Beowulf dies takes some of the hot air out of the exaggerated proportion ('bigger-than-life') typical of the epic hero stereotype. In the same breath, his death makes him more human, and therefore easier to identify with.