When Madhu reported seeing Sultan and talking to him nobody but RK believed her. She risked her life to expose the criminal but alas he was smarter than her and outsmarted not only her but also the special squad which was to help her capture the crook. When they realised they had the wrong man, RK's first reaction was to fear for Madhu's safety. This indicated that he did at that point believe that Sultan was alive and could harm Madhu who had gone back home alone. As feared Madhu was kidnapped and threatened. When she returned, she witnessed RK beaten up in the prison and hospitalised. At the hospital when Sultan called her to threaten her again she tried to alert the police but again Sultan hoodwinked her. When the lawyer tried to tell her to stop 'Sultan alive jaap' RK asked him to shut up but later told Madhu to forget about Sultan for her was dead and he had indeed killed him.
When the one man you love fails to believe you and the criminal keeps threatening to harm him if you utter a word to anyone, what are you to do? What is worse is although the criminal seems to be everywhere, no one is around to spot him, barring her. So basically she is on her own. With nowhere to go for help and no one to confide in, lest the criminal carries out his threat (she has seen him murder people in cold blood), she seems to have made up her mind to tackle this situation by herself.
So what is her plan? She is asking RK to grant her divorce, not only for Sultan's sake. In fact I think she is asking for it more for her sake rather than for Sultan. Am I confusing you? She has taken it as a given that Sultan will destroy their lives come what may. So she seems to have two alternatives - either kill the man or kill herself. If she does either of the two while letting RK still love her, she will destroy him too because the first option will see her in prison and the second will see her in heaven, leaving a heartbroken RK mourn the loss of his beloved Madhu. She loves the man more than herself. so she cannot bear to see him in pain. So she has chosen this alternative of demanding a divorce and giving the most ridiculous reason for it. She knows that the very name of Sultan will numb RK's senses and stir up all his anger against her. Once she gets the divorce from him, she is planning to execute either of the two options I have mentioned.That is why she keeps saying that Sultan may destroy their relationship but he can never have her.
Will RK discover the truth behind her lies and save her from self destruction? Keep watching Madhubala.
P.S. I am not one of the CVs nor do I endorse Madhu's harebrained plan if this is what it really is. Only trying to think as crookedly as Madhu (or should I say the CVs) is thinking right now.