I have a theory about some of this. With the knowledge that many white westerners have joined ISIS or attempt to join ISIS, I'm curious if they actually convince some of these people the same way that they convince suicide bombers that if they were to publicly execute them as "spies" that they could use the video as propaganda to massively fuel the cause of ISIS.

It's not much different than asking someone to strap a bomb to their chest, and westerners are fully aware of the power of media and might go along with this if they really do support the cause. As reporters have pointed out, when arriving at Raqqa you generally are vetted not only for security purposes but also for what abilities you have. If you are fervently religious but have no skills that are desirable, cannon fodder and propaganda tools are possibilities.

I'm not necessarily saying these particular westerners aren't spies, nobody will really know the truth about that except MI6 and ISIS themselves, but what if this has happened in the past?