Death is the end of all men.
One noticeable feature of the story is that the story of Sigurd and the Niblungs really only starts in the latter half of the book, but actually that's not exactly right. There's a sense in which the dilemma between right and wrong in Sigurd's story is actually the same as his father endured, and even his father's father Volsung. All these men have something in common—they die in the story. In fact everyone in the story dies. That's because everyone really does die, and in the end, all that's left is what people will say about us.
Legacy is the reward for heroism.
Legacy is the ultimate currency. Instead of fighting for riches like the story's cast of baddies, the heroes fight for honor and for freedom, and therefore, they manage to earn legacies for themselves. The heroism of Sigurd was his ability as a spouse. He was such a good man toward women that his downfall was two feuding women, and his legacy leads to the death of all his enemies. There is justice when this happens, and it's also Sigurd's reward for a life well-lived.
Selfish ambition only causes suffering.
Siggeir does not earn Odin's blessing, so he kills Volsung. That's the Cain and Abel story all over again. Gudrum's actions lead her to lose her husband, her wealth, her honor, her brothers—not to mention that innocent people die. Only bad comes from dishonorable or evil behavior.
