But it could be that ISIS burned al Kasabeh to death as an act of religious mercy. Dying by fire, not set by one's own hand, confers martyrdom on the victim. According to the Muslim site Islam Question and Answer,
The one who dies by burning is a martyr, because of the report narrated by Ahmad (23804), Abu Dawood (3111) and al-Nasaa’i (1846) from Jaabir ibn ‘Ateek (may Allah be pleased with him), according to which the Messenger of Allah (blessings and peace of Allah be upon him) said: What do you regard as martyrdom?” They said: Being killed for the sake of Allaah. The Messenger of Allaah (S) said: “Martyrdom is seven things besides being killed for the sake of Allaah. The one who dies of the plague is a martyr, the one who drowns is a martyr, the one who dies of pleurisy is a martyr, the one who dies of a stomach disease is a martyr, the one who is burned to death is a martyr, ...There has been over the centuries a great deal of discussion about what kind of death by burning exactly qualifies. The site quotes a scholar as saying that "in general," one who dies by burning is a martyr but that with very few exceptions (such as a firefighter), no such victim specifically can be so thought.
Muslims know all this, of course, which is why the outcry in the Muslim world at the killing has not been over its method but that it occurred at all.
Here is another example of the connection between death by burning and entry into paradise. "Honor killings" in the Arab/Muslim world are unfortunately all too common. The victim is a female (not necessarily a grown woman) who, in the opinion of her male relatives, has brought shame and dishonor upon the family by doing deeds such as adultery, fornication, being raped, flirting with a man or even becoming too westernized.
Burning the woman to death is a very common means of killing her. Example headlines:
"Yemeni girl, 15, 'burned to death by father'"
"A Father Burned His 13-Year-Old Daughter to Death for Walking Home With a Boy."
"Honor killing in Scotland: Muslim burns ex-wife to death for being “too Westernised"
"Man burns daughter to death in suspected honor killing"
There is nothing in the Quran that commands or even justifies honor killings, and Arab culture is certainly not the only culture that evokes such murders, but that burning is so common is almost certainly related to Mohammed's teaching that such a death confers martyrdom. Since martyrs are guaranteed paradise, killing by burning is thought to be less cruel, eternally, than by other means. This accounts, I believe, why in my readings of Jordan's responses to the pilot's death, I have seen condemnation that al-Kasasbeh was murdered, but not of the fact that he was burned. Jordan actually has a large number of honor killings each year, too, and burning is no less common there than elsewhere in the Arab world.
However, burning women to death is not even solely an Arab custom. "Bride burning" in India, usually over dowry issues, remains a huge problem there, accounting for at least 2,500 deaths per year.
Update: MEMRI has just published excerpts of a fatwa (Muslim religious ruling) issued by ISIS that states in part:
"Q: What is the ruling on burning an infidel with fire until he dies?"This statement does not directly state that the murdered pilot is a infidel, but that is clearly the implication (MEMRI did not publish the entire fatwa). If ISIS did indeed hold that al-Kasabeh was an infidel then his murder by fire was "retribution" for his sorties against ISIS.
"A: The Hanafi and Shafi'i schools [of Islam] hold that burning is completely permissible. They interpreted the saying of the Prophet that 'Only Allah shall torture with fire' as [a call for] humility. [The scholar] Al-Muhallab said: 'This ban is not [an actual] prohibition, but rather a means for [advocating] humility.'
"[Shafi'i scholar] Ibn Hajar, may Allah have mercy on him, said: '[This saying] indicates that it is permissibile to burn, as the Companions did. The Prophet blinded two men from 'Arina [whom he judged to be apostates and criminals] with a branding iron. Khalid bin Al-Walid, [one of the Prophet's Companions], also burned apostates with fire.'
"Some scholars hold that burning with fire is essentially prohibited, but is permissible while acting in retribution, as the Prophat did with the two men of 'Arina. He blinded them with an iron as an act of retribution, as is mentioned in authentic [hadith]. And this is the most prominent among the proofs."