What's the Deal With ISIS's Decapitation Inclination?

The Globe and Mail's opiner-in-a-hijab Sheema Khan claims that ISIS is a dead-ringer for a rogue group whose existence was predicted by Islam's founder:

Contemporary theologians have been drawing comparisons of the extremist beliefs and actions of the Khawarij to those of the Islamic State. The analysis illuminates the dangerous allure of this militant group, and Muslims’ duty to combat them by all means. The sword and the pen have now joined forces: Nations have mobilized to destroy the Islamic State, while more than 100 international Muslim scholars have issued a refutation of the group’s anti-Islamic ideology. 

The Prophet Mohammed had warned the nascent Muslim community about a sect that would emerge from Iraq; its followers would pray, recite the Koran, call for sharia and invoke God in persuasive fashion. Yet, the Prophet warned, they would have nothing to do with Islam, for their actions would betray the very fundamentals of the faith. In the name of God, they would kill those who did not agree with their interpretation. They would consist mostly of youths with “foolish dreams” of grandeur. And, if he were alive when they emerged, he would fight them, for they would be the “worst of creation.” 

The Khawarij emerged shortly after the Prophet’s death...

Wow, that was very prescient of him. There's a far simpler explanation for the beheadings, though, but it's one that true believer and propagandist-for-Islam Sheema would, for obvious reasons, prefer we ignore: the Prophet himself was into decapitation in a really big way. And for those who believe in the literal truth of Islamic doctrine, that can be reason enough to perpetuate the practice.