That's part of the tragedy of the story for me. Because of him being wrongfully imprisoned and having what was to be the best moment of his life ripped out from under him, Edmond (upon learning who was responsible) becomes solely fixated on revenge. So, by that point, there's no longer enough room for Mercedes in his heart, at least not until she comes to him pleading to spare her son. But his actions weren't for Mercedes, they were solely for himself.