Yazidis have a history of persecutions from Islam due to their belief in the Shaytan angel who is considered Satan by Islam. That is also the precise reason why ISIS hated them so much. As a group of terrorist who mainly believe in Sunni Islam running in the name of religious extreme, they naturally decided to express their detest by killing as much Yazidis as possible to "purify" their land, with raping, slavery, and torture as amusement on the way.