Imagine if the leader of ISIS announces to his followers that he had a vision from God and he told him that they must abandon Islam and become Christians, Jews or Hindus or whatever. They will continue to slaughter and kill, however from now on they will yell "Praise Jesus" or "Praise Whatever" instead of Allahu Akbar. What effect would this have on all Christians / Jews etc?
Do you think other Muslims will order these ISIS guys to be killed for converting to another religion, or will they stick to the story that ISIS is not real Islam so these guys where not Muslims in the first place?
Do you think after a while all the people of said religion ISIS converted too will be considered Terrorists? Or will those people only be considered terrorists if people get a feeling that they are turning a blind eye to the terror being committed by people of their religion?