ISIS have no time for these niceties, but there is no reason to believe that if the US backs off, leaves them in peace, apologizes for its own murders and agrees to pay a (whopping) compensation, with no strings attached, that ISIS could back down and come to a reasonable agreement with the now retreating and humbled kufar. That of course is unlikely at this point, but is the only solution unless the US wants to drop an atom bomb on the whole region, which bring Muslims and non-Muslims alike together to attack the US.
Remember the fairytale of Saddam Hussein killing babies on the eve of the 1991 Gulf War? All revolutions logically spill the blood of those who profess to be hostile to the new order, and the oppressor uses every trick in the book to discredit the revolutionaries carrying out their 'justice', and IS is no exception. In Cuba, between 500 and 2,000 enemies of the state were executed following the 1959 revolution, and was not swamped by US-backed protestors.
When will the US get it? The Middle East is in revolutionary upheaval, and has been for a century now. Just as the Russian revolution of 1917 was the logical conclusion of upheavals against imperialism in yesteryear, so Islamic revolutions are the logical consequence of imperialism today. The Iranian revolution of 1979 set the stage, and actions of the US in the past two decades (since the first invasion of Iraq in 1991) have ensured that this upheaval will continue and have an Islamic nature.
All revolutions result in revolutionary 'justice', seldom palatable to western secularists, who have long discarded revolution as something passe and slightly distasteful, unnecessary in our benign age of postmodern imperialism, where there are no guilty parties, where money is simply 'made' according to the law. '
But what is 'the law'? The law' in each land is refashioned by the new imperial dictator to meet the needs of the economic order, where all economic activity is condoned as long as it is carried out in conformity with 'the law'.
The driving force behind this 'law' is exploitation in pursuit of unlimited profit, leading to world wars which leave humanity and nature in peril. The 'law', for instance, includes condoning gambling (both explicit and metaphorical in the form of the 'stock market'), and regulating various forms of moral degeneration (prostitution, drunkenness)--all anathema to Islam. All the while, legal recourse against crimes committed by the most powerful in this world order is impossible, given their control of the economic and hence political processes, and indeed, the pious United Nations, which lives on only with the sufferance of the Dragon.
It is this enforced ascendancy of economic power over the popular political will that makes 'political Islam' necessary today, after the defeat of the communist resistance to capitalism. Nothing short of a new 'law' will do, where a code of ethics is embedded. The communist revolutions for the most part failed to achieve this and Islamists became the main force of resistance to imperialism by default.
Islamists learn from the imperialists
The end of communism came by the imperialists wearing down the secular communists through war and subversion until the system was sufficiently weakened, allowing internal dissent to overthrow it. This clever 'strategy' was learned by Bin Laden, and despite all the pathological killers in US-Israeli forces, figleafed by learned Princeton scholars, this seems to be the reality of politics today. The man in an Afghan cave is destroying the empire.
If you don't believe me, just keep in mind that Islamic resistance, based on martyrdom in the defense of religion, is grounded in a much more convincing way than was communism. Post-9/11 CIA counterterrorism head Robert Dannenberg admitted: "It was US officials continue to be stymied by ISIS/IS. General Nagata's Strategic Multilayer Assessment Committee at the Pentagon called on academics to help out, including "business professors examining the Islamic State's marketing and branding strategies. We have not defeated the idea. We do not even understand the idea." Much easier to convince a Soviet that your way of life was better. You could take them to Kmart in the US or to Wal-Mart, because they were driven by many of the same things that we're driven by." Ex-CIA director George Tenet notes in his memoirs, "Al-Qaeda boasts that while we fear death, they embrace it."
As for IS's reported atrocities, given IS's commitment to sharia and the Quran, those Christians and Shia caught in the (Sunni) IS order--if they value their lives and want to remain in their homes--must organize and present themselves to IS leaders as loyal citizens, opposed to IS's enemies--the West, Israel and Saudi Arabia. Sharia makes allowance for the protection of non-Muslims, who must pay a protection tax (jizya). That will end the sectarianism.
At present, these dissidents are sitting ducks, and the crocodile tears of western leaders and the media are making things worse for them. This cynical use of dissidents has long been ammunition in the western arsenal to confront imperialism's enemies, as the Soviet Union experienced. They are custom-made martyrs for western media--and quickly discarded when the West's armies and carpetbaggers are welcomed as liberators.