Although not necessarily considered part of mainstream Islam, everything ISIS does is according to Sharia and the example of Muhammad. So, ISIS members have the required qualities of Muhammad's example and meet the strict standard of Sharia. Hence, they can be spoken of as 'good' Muslims.
Okay. So we agree that on ISIS interpretation of Islam, rape is fine and dandy in some cases. What you have demonstrated so far is therefore that on ISIS interpretation of Islam, which you yourself describe as not necessarily mainstream*, rape is fine. Accordingly you implicitly agree there are other interpretation of Islam according to which rape is not okay. (you agree with me on that? or do you believe all interpretations of Islam teaches its adherents they *have* to rape?)
At most where you end up is that you work with a definition of "good muslim" according to which ISIS are PART of the set of good Muslims and ISIS allows rape (I refrain from parsing the difference between allows and orders rape, i.e. the "have").
Can you please tell me how you get from these premises to the statement that a "good Muslim" have to rape?
For your ISIS example to be relevant you will have to prove ISIS contains all the "good Muslims". I know ISIS believes that, but I am very curious as to how you will prove it.