Why is it significant that the end of Beowulf's life was caused by the actions of a slave?

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I think that the significance has to do with the fact that if you really look at the story, Grendel was just a large, strong, homeless man; with apparently some serious jealousy issues. Beowulf, talked so highly of himself; i really feel that the significance comes into play when Unferth tells him about Brekah. Another thing Beowulf talks highly of himself when it comes to fighting and how strong he is, he thinks he will never be defeated, until this point apparently. Hrothgar believed in Beowulf, and Beowulf's men believed in him as well. I really think he had gotten to the point when all he was doing was saying how strong he was instead of really putting that into action, his emotions became defeated, which in return, made him as a warrior defeated. He talked a big game, but in the end, his arrogance and himself were put into their place by realizing that he needed to be defeated in order to see that there is more to life than talking a big game about yourself. This is my point of view, take it for what it's worth, and if you have any feedback for me, let me know.