Eh, I don't know if an IS execution is comparable to a Saudi one. A guy gets decapitated for criminal conduct in Saudi, it is a pretty quick process and they make sure the decapitating blow is swift. These insurgent groups add in an extra element of extreme cruelty, suffering, and torture to their executions. They subject their captives to torture and other acts that are forbidden by their own religion. Using fire as a means of punishment or execution is forbidden in Islam, and is a torment that they believe is reserved for God's use only.
These people are involved in drug trafficking as well, making them criminals under the Shari'ah. The use of intoxicants is a serious crime alone. Trafficking narcotics is a crime punishable by possible execution, crucifixion, exile, and/or the right hand and left foot being amputated. Though each case may differ and the offender might spend several years in prison with a set amount of lashings being prescribed as punishment as well.
These "terrorists" are viewed as just being more extreme in their beliefs than mainstream orthodox salafism that the majority of Saudis follow. Except this is not really an accurate way to perceive them, they aren't "more extreme", they are completely deviant in their methodologies and applications of the Shari'ah. They condone executions by fire and approve of torture, things that are forbidden. And if that isn't convincing enough, they sell fucking narcotics and work with criminal organizations. Then they go about slaughtering orthodox Salafi Muslims who refuse to pledge allegiance to their mysterious caliph, even though they know they will be savagely executed.
IS claims to be upon the salafi methodology, yet their actions contradict their claim. They have declared war upon the Saudi royal family, who supports salafism, and they brutally murder salafi Muslims, take their property, and abuse their women, all because they refused to accept their caliphate and refuse to join their cause.
I hear these supposed "experts" speak and they refer to these people as Wahabists and no real expert should be using that term because that is not a term they use to refer to themselves. And listening to these people they put both the mainstream salafists, loyal to the house of saud, and terrorists under this same Wahabi umbrella. They go on and try to explain that these two groups are very similar and only differ slightly and it is in regards to rising up against their leaders. While this is true, there are many other differences that clearly set these people apart. And there are many other major differences between these people. Someone who claims to be an "expert" and uses the term Wahabi and clumps these two factions closely together is no expert at all.