The atrocities for which ISIS has become known are typical of a functioning Caliphate. The execution of Muslims who do not submit to the Caliph, the ethnic cleansing and sexual slavery of non-Muslims are not aberrations. They are normal behavior for a Caliphate.
The last Caliphate, the Ottoman Empire, was selling non-Muslim girls as sex slaves after the invention of the telephone. A New York Times report from 1886 documented the sale of girls as young as twelve, one of them with “light hazel eyes, black eyebrows and long yellow hair”. An earlier report from the London Post described Turks, “sending their blacks to market, in order to make room for a newly-purchased white girl”. This behavior is not a temporary aberration, but dates back to Mohammed’s men raping and enslaving non-Muslim women and young girls as a reward for fighting to spread Islam.
The ISIS behaviors that we find so shocking were widely practiced in even the most civilized parts of the Muslim world around the time that the Statue of Liberty was being dedicated in New York City.
To Muslims, the end of slavery is one of the humiliations that they had to endure because of the loss of the Caliphate. Europeans forced an end to the slave trade. The British made the Turks give up their slaves. The United States made the Saudis give up their slaves in the 1960s. (Unofficially they still exist.) When the Muslim Brotherhood took over Egypt, its Islamist constitution dropped a ban on slavery.