If I were your teacher and you gave me the following answer I would mark it correctly, because it is an interpretation that strongly connects to the facts, yet, this could not possibly be a fact-based discourse simply because the story does not present such an opportunity.
I would say that it IS a pre-ordained and expected outcome for a simple reason: This is an epic story where the protagonist is some sort of tragic hero who, in the end, will succum to his weaknesess and, in the way, he will demonstrate how every human being, leader or follower, master or servant, can equally succumb to demons that would have the power to destroy all their good deeds.
The background validity of Beowulf is complex, because this debated piece of literature was transcribed by early scribes AFTER it was transmitted orally through folkloric means. Its epic nature entails that it will be a story where many things may, did AND would have taken place but we can only assume they did depending on how strongly we choose to accept the information.
The characters in Beowulf, at least most of them, were also mentioned in other epic stories, which leads us to believe that Beowulf is a generalized, yet patriotic attempt to bring a nation together in times of war with Saxons in Scandinavia. (I.E, these were very rough times, and the Scandinavians needed to reaffirm their status as citizens and soldiers).However, bringing Beowulf as an antihero or as a tragic hero leave us to wonder exactly why the Danes chose to portray their own story in such a dramatic way. It is not a unique case: The Spanish did the same with countless chivalry novels in the late Spanish medieval literature and early Spanish Renaissance. So we might see a literary trend that may lead us back to the scribes who may have added (or not) elements of modern guilt-ridden religious nature to the former "super" heroes of ancient lore.
One thing is for sure: The outcome of Beowulf and the pre-ordained tragedies that would surely define him as a character are part of the richness of this epic novel, and are ultimately what connects a reader from any generation to it, making it universal, enduring and maybe even a bit magical.