Why doesn't Edmund Dantes marry Mercedes at the end of "The Count of Monte Cristo"?

In Alexander Dumas’ 1844 novel The Count of Monte Cristo, Edmond Dantes has suffered betrayal of the worst kind, and paid a price in bondage under horrible conditions that have no place in civilized society, then or now.  He has endured unspeakable horrors during the years of imprisonment, and succeeded in regaining his freedom and wealth.  Dumas’ story is bleak; references to misery and happiness as existing on equal terms abound throughout the novel.  In his letter to Maximilian, whose own life has been akin to a Shakespearian tragedy, Edmond, as the count, writes:

“There is neither happiness nor misery in the world; there is only the comparison of one state with another, nothing more.”

Dantes understands that the tragedies of his existence, irrespective of the wealth he now enjoys and the revenge he has succeeded in extracting, have robbed him of more than time and a life with the beautiful Mercedes.  He has caused harm, in particular, to Albert de Morcerf, the dutiful and admirable son of Count de Morcerf, in actuality Fernand Mondego, the “friend” who betrayed him, and is convinced that his methods have not spared the innocent.  Albert’s family name is destroyed by Edmond’s machinations and the young man believes the only way to resurrect the family name is through a demonstration of his own courage in battle, towards which end he departs to join the army in Africa (“I am to leave for Africa, where I will earn for myself the right to use the name I now bear, instead of the one I have thrown aside”).  That Mercedes reveals to Edmond that Albert is actually his – Edmond’s – son, and not the now-deceased Count de Morcerf’s only serves to ensure that no union between Edmond and Mercedes is possible.  The great and tragic irony of the story, of course, is that Edmond’s list of intended victims had included Mercedes, whom he believed betrayed him also because of her marriage to Fernand Mondego.  Now, with Fernand dead and Albert gone to try and save the family name, Mercedes is a broken woman, emotionally and financially.  When Edmond encounters her near the end, his observation of this once fine and joyful woman is as follows:

“. . . no, the change in Mercedes was that her eyes no longer sparkled, her lips no longer smiled, and there was now a hesitation in uttering the words which formerly sprang so fluently from her ready wit.” 

Mercedes refuses to marry Edmond because she is convinced of her guilt in the chain of events that have led to this day: “You have spared me, yet of all those who have fallen under your vengeance I was the most guilty.”  And then, “I perceive that you are intending to propose to me; but I cannot accept it, Edmond – my son would not permit it.”  Mercedes understands that there is too much pain between them, and that Albert’s pride, if it doesn’t get him killed, will never allow for a union between his mother and the man who destroyed his family’s good name.

Why couldn't Monte Cristo marry Mercedes in "The Count of Monte Cristo"?

For all that they've been through a lot together, Edmond feels that he cannot marry Mercedes, the woman he'd loved for so long. Primarily, this is because he no longer harbors the same feelings for her as he once did. For her part, Mercedes has never stopped loving Edmond, but one senses the intensity of her feelings aren't reciprocated. It is very difficult for Edmond to have much respect for Mercedes, let alone love, after she marries the dastardly Fernand.

The very sight of Mercedes causes Edmond to tremble, as she undoubtedly reminds him not only of the love they once shared, but also of the immense trauma he suffered in the past. Edmond may have successfully carried out his elaborate plan of revenge, but he still cannot escape from the ghosts of the past, which continue to haunt him day by day. Edmond simply has too much emotional baggage and must make a clean break with the past if he's to move on with this life. Among other things, this means that he cannot be with Mercedes.