According to this article, ISIS fighters were executed by members of another extrimist jihadi group in Syria. These other jihadis also have a sense of ironic humor: they dressed up in orange jumpsuits, like the ones ISIS's beheading victims wear, and killed ISIS fighters dressed in black, the usual combat and execution uniform.
Obadiah 15 contains a principle of divine justice that my father emphasized to me many times as I was growing up:
As you have done, it shall be done to you;
your deeds shall return on your own head.
So also James says: For judgment will be without mercy to anyone who has shown no mercy (Jas 2.13).
Perhaps some of these fighters beheaded Christians or Yazidis or others before being exterminated themselves. They chose a life of violence, a life of the sword, and they fell victims to this very same life. They got what was coming to them, at the hands of other persons just as wicked as they are.
If you commit violence and kill, you will end up a victim of violence and will be killed. That is why in scripture we also find the injunction to be merciful: Blessed are the merciful, for they will receive mercy (Mt 5.7). A person who acts from goodness and from benevolence knows that eventually, he too will get good, even if for the moment he should suffer. This is the justice of God's world: you choose the kind of life you get, one fitting given the choices you make and the values you uphold.