Quran states: "You must obey everything in the Quran (without question) otherwise you're a hypocrite."

Quran 33:36

It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair. And whoever disobeys Allah and His Messenger has certainly strayed into clear error.

Quran 4:65

But no, by your Lord, they will not [truly] believe until they make you, [O Muhammad], judge concerning that over which they dispute among themselves and then find within themselves no discomfort from what you have judged and submit in [full, willing] submission.

Quran 4:150-151

Indeed, those who disbelieve in Allah and His messengers and wish to discriminate between Allah and His messengers and say, "We believe in some and disbelieve in others," and wish to adopt a way in between - Those are the disbelievers, truly. And We have prepared for the disbelievers a humiliating punishment.

Quran 4:142

Indeed, the hypocrites [think to] deceive Allah, but He is deceiving them. And when they stand for prayer, they stand lazily, showing [themselves to] the people and not remembering Allah except a little,

And if you're a hypocrite:

Quran 9:73

O Prophet, fight against the disbelievers and the hypocrites and be harsh upon them. And their refuge is Hell, and wretched is the destination.

You must be killed.

https://adragons.github.io/1/sophism.html#hypocrite

Edit:

This means groups like ISIS are not "fake" Muslims. They have theological justification, and seem bound to appear.