In Chapter IV Dantes remarks that happiness can only be attained "by overcoming "dragons [that stand over one's happiness]...."  The dragons that intervene between one's happiness and one's fates are each person's character flaw.

The fates of each character, then, depend upon whether they are able to conquer the dragons of their character.  For instance, Danglars's flaw is his avarice;  failing to conquer his greed, he rejects his wife who has lost sums of money; he seeks to regain money even if it means selling off his family member.  As a result, he loses what really matters: family and his life.

Similarly, de Villefort loses his family because of his dragon:  He seeks power even if he must hide his Bonapartist father and bury his illegitimate son in order to maintain a prestigious reputation.  Because of his deception, he is exposed by Dantes, with tragic results.

Fernando's desire to attain prestige and military position leads him to kill and to betray the trust and love of a Middle Eastern king. His dragon of treachery turns itself on him and he, too, is exposed as the "hyena" that he is.

Only Edmund Dantes overcomes his dragon, his desire for revenge.  He, then, is redeemed by saving the Morrel family and helping a young couple save their love.