The reason for the tragic end of the hero Beowulf can be attributed to several different factors.
Beowulf is too confident in his own strength and abilities. In one particular instance, he faces a fire-breathing dragon without any weapons and without any armor to protect him. He defeats the dragon by ripping off its arm, but he is injured in the process.
He underestimates the power of his enemies. When Beowulf returns home from his battle with Grendel’s mother, he finds that his king has been murdered by another king named Unferth. Beowulf doesn’t realize at first that Unferth is behind this crime until it’s too late—Unferth has already killed Beowulf’s wife as well as many other people who were loyal to Beowulf.
He is too proud and doesn’t listen to advice. When Wiglaf offers his help after seeing how badly injured Beowulf is from fighting Grendel’s mother, he refuses because he doesn’t want anyone else getting credit for what he considers his personal glory. Wiglaf then goes on to defeat Grendel himself before dying alongside.