The Islamic State has become famous for their execution methods and this has sparked many questions.
One of many is "Why would they do this?"
To answer this question we have to understand one of the basics of Islamic law, Qisas.
Qisas is defined as retribution (although there is no perfect english definition).
In the english language this type of law would best be described as "An eye for an eye"
The proof that the Prophet pbuh prescribed and carried out Qisas punishments is numerous.
O ye who believe! the law of equality is prescribed to you in cases of murder: the free for the free, the slave for the slave, the woman for the woman. But if any remission is made by the brother of the slain, then grant any reasonable demand, and compensate him with handsome gratitude, this is a concession and a Mercy from your Lord. After this whoever exceeds the limits shall be in grave penalty.
Surah Baqarah ayah 178
It is important to not here that this verse does not mean that if someone kills your slave that you may kill that person's slave. This was something that was practiced in the time of Jahiliyya (time before Islam in Arabia) and was banned by the Prophet pbuh because it causes harm to someone who did no crime. Rather it means that the one who committed the crime will be held accountable.
Narrated Anas: The daughter of An-Nadr slapped a girl and broke her incisor tooth. They (the relatives of that girl), came to the Prophet and he gave the order of Qisas (equality in punishment).
Sahih al-Bukhari, 9:83:32
This clearly shows the Prophet pbuh using Qisas as a justice.
This is generally the principal IS uses in order to justify it's executions.
In the video of the soldier getting driven over by a tank, he confessed to running over IS soldiers while he drove a tank for the Regime, so IS used this principal to execute him in the same way he killed IS soldiers.
The most famous version of this used by IS is the burning of the Jordanian Pilot.
The way IS justifies it is Qisas because the pilot had burned people alive in building because of his bombings.
This has proven controversial for many reasons.
Mainly because of this Hadith:
“Indeed, fire is something that no one other than Allah may use for punishment.”
Sahih al-Bukhari (3016)
This has called many Muslims to call IS's actions unislamic and condemned them for this act.
IS argues that because this is a case of Qisas, this was justified. They also cite the Hadith that Ali (ra) burned heretical rebels as a way of execution, which was not even in a case of Qisas.
Ikrimah relates that some heretical rebels were brought before Ali (ra) and he had them set afire. When news of this reached Ibn Abbas (ra), he said: “If it had been up to me, I would not have burned them, because of Allah’s Messenger (peace be upon him) prohibited this, saying: ‘Do not punish with Allah’s punishment.’ I would have merely executed them…”
Sahih al-Bukhari (6922)
This is a weak justification for their actions for many reasons
Firstly, it is possible that while Ali (ra) burned the people, he may have not been present when the Prophet said not to burn people. So while he did it, he did it out of ignorance of the Prophet's statement, and because this statement is now well known, it is no longer justifiable.
Second, there are many discrepancies within this story. Some narrations say that it was actually their houses that were burned due to blasphemous material contained within the houses. Others say that they were executed and then their bodies were burned after the execution had taken place.
These stories are in Ibn Hajar's book Al-Fath Al-Baari Kitaab Istitaabah Al-Murtaddeen