Nothing says it better than this:
"Fool that I am...that I did not tear out my heart the day I resolved to revenge myself."
The Count of Monte Cristo features a man who gives way to bitterness and hatred, holds the greatest grudge ever known to mankind, and eventually loses himself in revenge. Everything he was before, all of his innocence is now gone, not necessarily because of his misfortune, but because he chose to give it all up and become someone else--he chose to be hateful.
"Hatred is blind; rage carries you away; and he who pours out vengeance runs the risk of tasting a bitter draught."
This proves to be painfully true for the man who was once Edmond Dantes. In his bitterness, in his insistence to hold a grudge and get revenge on those who wronged him, he loses himself. He acts the part of God, doling out mercy and justice as he sees fit. He loses his chance at the happiness that was once available to him.