When his world was destroyed, Maleficent gave him a chance to live by allowing him to join the Organization to which he quickly obliged. Why he did so without hesitation is currently unknown. He says it's because he wants to further torment Nick, but it could possibly that he was manipulated by Maleficent's dark powers. Another reason he could have joined was because his estranged little sister, Shannon had joined the Organization, and they promised to protect her. But later down the line, Marlon learned the real truth about the Organization's deception and went against them. Marlon went off on his own along with Shannon. The Organization continues to hunt the pair.
Despite this, he refuses to work with the Society nor is he on good terms with Nick or any of his friends. But from time to time, he does manage to save the day, but mostly for his, and his's sister own interest. It's not known if Marlon can change, but there might be hope.