I am struck by the similarities between IS treatment of Yazidi, Christian and other minority groups in Northern Iraq and that of the Christians and Muslims of Palestine by the Zionists in 1948-9. The one difference is that the Palestinians were not offered the option of conversion to Judaism. But the technique of threats followed by public assassination leading to widespread terror and flight is well documented, see e.g. Ilan Pappe’s ‘The Ethnic Cleansing of Palestine’.
One other difference is that the Zionist armed groups were happy to operate an ‘arms length’ policy, giving freedom and flexibility in interpretation of strategy to commanders in the field. That practice of ‘deniability’ still operates in the Israeli military. Soldiers know their responsibility will be hard to pin down. In the event a charge is made progress through the legal process will be so slow that the eventual ‘not guilty’ will barely be heard.
As to the leaders, whoever they are, recent history suggests that, were a stable ‘Islamic State’ to emerge somehow in the next ten years, it would have a leader ‘we can do business with’. All in the proud succession of Gaddafi, Saddam Hussein, Begin and Sharon, (a controversial and much abbreviated list)