All Muslims must stand together and not only condemn them but work together to defeat extremists like ISIS. Some critics of Islam blame the strict interpretation of Islam by Wahhabism a Saudi strand of Islam and the teaching of children to curse and hate kuffars for the rise in fundamentalism and extremism.
Peace, like democracy, cannot be imposed from above, or from outside. But if the two sides in Syria's civil war can agree to at least a few temporary local truces, they may be better able to turn their attention to IS. That's certainly what would be in their best interests, and in the interests of their foreign backers, whether Iran, Saudi Arabia, Qatar or Turkey.