In the movie, Fernand has a gambling problem, which Dantes uses to drive him to ruin until he's deep in debt. Later, when Fernand has nothing left to lose, he challenges Dantes to a duel, hoping that if he doesn't die first, he'll win it all back. In other words, Fernand's gambling problem comes full circle when he takes one last gamble.
Also another Fatal Flaw of Fernand that comes back to bite him: his petty, childish envy towards Dantes. Rather than run away to survive and try rebuilding his life and fortune, he'd rather try killing Dantes because, as he puts it himself...
Also, his final defeat and death perfectly mirrors the first scene we see of him post-Time Skip: him apathetically dueling an aristocrat wanting revenge for Fernand sleeping with his wife. Fernand callously kills the man with a stab through the heart, then mocks his Honor Before Reason righteous indignation and vengeance attempt towards his actions, the fact the man's family and wife are ruined from the scandal (plus his death) AND Mercedes for being upset about it all. His Karmic Death comes at the hand of the first man and best friend he cheated out of a wife, after said wife cheated back on him and rekindled her relationship with said man, on top of him destroying Fernand's wealth and reputation. In a Duel to the Death where he's stabbed through the heart, because he also couldn't swallow his own pride and accept Dantes' victory and mercy.
Remember how one of the things that the Priest taught Dantes was "Neglect shall be our ally"? Unintentionally, it also applies to Fernand's relationship with Albert as well. First, if he hadn't permitted Albert to essentially travel to Rome alone, then Dantes wouldn't have had to rescue the boy ( albeit in a staged fashion), thus allowing him to be invited to his birthday. Second, if Fernand had come to his son's birthday (instead of putting it aside to talk with Villefort about stealing the Count of Monte Cristo's money), Dantes wouldn't have given that speech that made himself a father figure in Albert's eyes. Looks like Parental Neglect was also Dantes' ally.
Additionally, it was also Fernand's neglect of Mercedes (in favor of his four other lovers) that also made it less difficult for her to choose Dantes over him.
