Peter Kassig, from what I am reading, was a politically engaged young man who wanted to help. He converted to Islam in 2013 after being kidnapped. What has happened to him and the 12 other Syrians, if the authenticity of the video is confirmed, will also reaffirm ISIL's position on Islam.

There is a train of thought in Islam that suggests we are all Muslims when born, and we do not convert, we simply revert.

Using that thought, Peter and other Syrians allegedly killed were actually always Muslim. So why execute other Muslims? ISIL have said that Kassig was an Iraq war veteran, and the Syrians were soldiers so therefore it was religiously justified.

The reason can be found in the obscure term "apostate." An apostate is someone that either renounced or abandoned religious principles or faith. In Islam it is further ‎defined as someone who has denied his natural instinct to submit to God.

As a convert to Islam, Kessig found his natural instinct for God, hence the whole revert/convert idea. ‎

For ISIL, apostasy is a grave sin, perhaps the gravest. In my reporting on the deaths of Kassig and the 12 Syrians, I posted some lines on social media.

Here is the full text: "#ISIL say that Peter Kassig was killed because he was a veteran of the Iraq war. He was killed for coming to a Muslim land to kill Muslims. He converted to Islam, so I guess he was in fact a Muslim killed by Muslims for coming to a Muslim land to help Muslims. Religion has little to do with this. Retribution and territory have more."

Immediately, I was mocked by ISIL supporters who called me an apostate and also referred to Kassig and the others as such.

In fact one ISIL supporter, with his twitter profile showing a fighter with his face covered, toting an AK47, said: "We don't judge it that way. Anyone working with the apostates is considered one of them."

And there lies the crucial ideological difference between those that support ISIL and those that don't and consider themselves Muslims.

ISIL have a narrow, clearly defined role for those that pledge allegiance to the Caliphate. It is their way or no way at all. No freedom of thought or expression outside of that.

For them Islam is about submission and therefore they believe Abu Bakr al-Baghdadi, ISIL's leader, is the rightly guided Caliph and to be followed without doubt.

But with that rigidity comes an absolute moral standard and that is dangerous. Beheadings become routine. Accusations against anyone who is not with you become commonplace, and the penalty for apostasy is death. ISIL have a clear idea of what being an apostate is. Anyone that is not one of them or abides by their laws.

But remember it is not just ISIL who think in absolutes. The Bush doctrine, named after the US president George W Bush, was a version of the same idea, "you're either with us or against us."

Considering he was the architect of the Iraq war, that was the starting point for where we are now.

It is an indication of where an inflexible doctrine can lead. ISIL have a powerful ideology that attracts supporters because they believe it is the right path. No amount of shock at their tactics will destroy that idea.

Follow Imran Khan on Twitter @ajimran