I highly doubt that a raper can be at the same time a believer. Of course in his own eyes he can be one, but you are not a Muslim just by saying you're a Muslim. Dying in the state of Islam, while being a Muslim is not easy. There are rules to follow. You can't just write Muslim on your forehead and run around doing crazy stuff. So you can't compare a rapist to the worst Muslim. And I highly doubt that someone can live a perfect life according the Quran and remain a disbeliever. Let's take for example a son that ignores his mother for no reason, like she never existed, but he is good to other random people. Can this person be good? He is unjust to someone who gave him birth and raised him. What are his good deeds worth after all. So, when you believe in God, you also believe that He is Creator of everything. Every single cell in your body. You can see, hear, feel, think, enjoy and all because of Him. Do you feel how unjust one is by disbelieving in Him?