Fernand is the worst of the all conspirators
I know that Danglars may seem worse, but he is so basic, he's all about money. Fernand on the other hand is a REAL piece of shit. He's the one who send the letter, then he spends months lying to Mercedes about Edmond, next he betrays Ali Pasha, and sold Haydee and lastly Mercedes was clearly forced to marry him, because if she dosen't do that, he would be starving to death.