The sins of a Muslim does not give the modern day khawarij any authority to jihad upon muslims. The khawarij are to be killed like the people of Ad and thamud were destroyed.
The khawarij are not authorities to decide who committed kufr or not and what actions should be done in response..there are Islamic scholars and those in authority who will analyse that by due process and there are shariah way of correcting it if ur allegations of kufr has some truth in it. Not wearing a hijab and having a non hijabi girls poster on a street does not make one a kafir.
Their feeble outward show of adherance in some relatively much less important issues is what I called namesake shariah. Like the hadith says, things don't enter beyond the throats of the khawarij. And by self styled I referred to their dubious naxalite and bandit style means and dubious authorities they take opinions of shariah from.
What would happen is the same story. These young fools who are tools of the dajjal, would come up with some weird, stupid and bedoin understanding of shariah that would drive the local population much averse to shariah and the leader of the khawarij, the dajjal, would enter to drive people to the other extreme of apostacy and atheistic secularism.