As far as Qur'an is concerned , burning people to death is the practice of enemies of ALLAH swt , that is the end of the story . Whomever practiced it followed the path of Satan .
Qur'an mentioned different ways how to treat a captive . If IS are true Muslim (!?) , they could have thought those verses that ALLAH swt mentioned in the Qur'an before burning a Muslim captive to death .
2.85 After this it is ye, the same people, who slay among yourselves, and banish a party of you from their homes; assist (Their enemies) against them, in guilt and rancour; and if they come to you as captives, ye ransom them, though it was not lawful for you to banish them. Then is it only a part of the Book that ye believe in, and do ye reject the rest? but what is the reward for those among you who behave like this but disgrace in this life?- and on the Day of Judgment they shall be consigned to the most grievous penalty. For Allah is not unmindful of what ye do.
8.70 O Prophet! say to those who are captives in your hands: "If Allah findeth any good in your hearts, He will give you something better than what has been taken from you, and He will forgive you: for Allah is Oft-forgiving, Most Merciful."
76.8 "They perform (their) vows, and they fear a Day Whose evil flies far and wide. And they feed, for the love of Allah, the indigent, the orphan, and the captive -- (Saying), 'We feed you For the sake of Allah alone: No reward do we desire from you, nor thanks.'