Extremism should not be linked to religions. An extremist is an extremist regardless of his/her faith. Religions don't teach extremism. Its the person that use religion to justify an action and should be called an extremist person but not an Islamic extremist. We should be civilized and learn how to classify people accordingly without creating labels that incite violence and create clashes among people?
Of course extremism must be linked to religion. Particularly when a religion's primary text, propagates extremists ideology and belief. Do you not consider stoning people to death and cutting off people's hands extremism? the claim that religion cannot be extremist in nature is frankly ludicrous and insulting.
The Flag/ banner ISIS uses is that of Prophet Mohammed PBUH which ISIS hijacked so rising the banner doesn't mean supporting ISIS.
That is false. The Seal of Muhammad on the flag, is perhaps a neutral Islamic symbol, but the black flag with white writing on top, be it the Shahada or the Seal, is indeed considered a Jihadi artifact. There were no black flags with these symbols and writings on them during the time of Muhammad, and today they are exclusively adopted by terrorists and Islamist groups as well as their supporters. The so called banner of Muhammad is a black flag with nothing on it.