There appears to be widespread confusion in the media about who the Yazidi are, what they believe and why they're persecuted by ISIS. I long have had an interest in the many religious origin stories, so I'd like to share my thoughts on the matter.
The Yazidi believe that Melek Taus, the "Peacock Angel" was the first being created by God as an emanation of God himself. God told him to not bow before anyone but God himself.
Later God created the lesser angels out of Gods light. One day, God ordered the angels to collect dust from the earth, out of which he created mankind.
When God ordered the angels to bow before man, the "Peacock Angel" refused, because God had told him not to bow before anyone. He said "How can one that is an emanation of God bow before mere dust?"
The Yazidi believe this was in fact a test, and because of his refusal, the "Peacock Angel" was put in charge of all angels and of all of mankind.
The problem is this: In Islamic beliefs, when God ordered the angels to bow before man, one of them refused. This angel, Shaitan, was punished for his disobedience, and cast from heaven and became Satan.
Throughout history, many Muslims have misunderstood these two origin stories to be the same, and therefor thought of the Yazidi as devil worshipers, possibly the worst sin in Islam, which has led to centuries of persecution.