Beowulf is able to defeat evil in the form of Grendel and Grendel’s mother, yet he loses his life when he battles the dragon. What theme does this suggest about the struggle between good and evil?

It can be hard to assign values such as "good" and "evil" to the events in Beowulf. For Beowulf, "good" is that which gains him glory; the point of a warrior’s life is to perform great deeds that will be remembered. This, after all, is what brings him to Heorot in the first place—the opportunity to show himself to be a man of great strength and valor. Beowulf’s subsequent reign over the Geats is successful in that his reputation has secured them from invasion.
In a way, his final battle with the dragon is a fitting end to his story. It is far better for Beowulf to go out in battle defending his people than to die quietly in his bed. Beowulf’s death, however, is a great evil for the Geats, who now are open to attack from their enemies and lack a great warrior to defend them.

A possible theme would be that the struggle between good and evil often results in a Pyrrhic victory.
The word "Pyrrhic" refers to the results of the conflict, where the victor's losses are almost as great as those sustained by the conquered. In a Pyrrhic victory, there is the sense that the victor's gains have been nullified by the extreme losses incurred. If you're interested, please refer to the link below about the origins of the phrase "Pyrrhic victory" and a history of such victories on the battlefield.
In Beowulf, the protagonist's final victory is won at great cost to himself. He kills the dragon but sustains a fatal wound. In his final moments, Beowulf voices his regret that he has no son to bequeath his legacy to. Yet, he comforts himself that, during his fifty-year reign as king, no other monarch dared to openly challenge him in battle. During his last moments on earth, Beowulf also rejoices that his death will not leave his people helpless; the dragon's hoard will enrich his subjects. So, the conflict between Beowulf and the dragon suggests that evil is often defeated at great cost to the good.
https://www.warhistoryonline.com/history/thermopylae-bunker-hill-and-2-other-pyrrhic-victories-throughout-history-winning-isnt-always-worthit-m.html