It's a fundamentalist, very radical take on Islam. It is believed by a small number of people who have been brainwashed because certain things have meant that they are easily brainwashed; whether that be extreme poverty, grief or something which has meant their life needs resolving in some way. The people doing the brainwashing are power hungry and driven by the same mental complexities shared by dictators.
It isn't brainwashing, at least not in the traditional sense.
The rise of fundamentalist / "back to original Islam" types is almost a cliche in Islamic history - if you look at Muslim Spain for instance, you have the initial Umayyad conquest, which was followed by vast wealth, which was followed by a weakening of faith and society, which was followed by the replacement of that regime by fundamentalists (the Almoravids), which was followed by vast wealth, which was followed by a weakening of faith and society, which was followed by the replacement of that regime by fundamentalists (the Almohads), which was followed by vast wealth, which was followed by a weakening of faith and society, which was followed by the
reconquista. Such is the esteem in which the Prophet and the first generation of Muslims are held that its easy to hold them up against a current ruler and see all the latter's faults in terms of following God's guidance.
All that has happened this time is that the fundamentalists have been artificially boosted - beyond anything seen before - by being key allies of the West. For all their anti-Western rhetoric, the reality is that Wahhabists have consistently sided with
and been supported by the dominant Western powers of the day - first us, and then the Yanks - in exchange for access to their oil. We (and the Yanks) actually spent nearly a hundred years encouraging the feudal / religious side of Arab life in opposition to the nationalist / modernist side; hence why even after 9/11 we have spent far more time going after secular Arab regimes (and threatening Shia regimes) than actually dealing with the people who attacked us.
Combine that with their almost unimaginable wealth, a world that is even now largely at peace and modern communications technology and they both have no reason to change their views and have the means to spread those views worldwide. Of course, those views are largely daft - for instance its hard to believe in the sanctity of a belief system
that trashes the house that the Prophet was born in whilst also turning Mecca into something akin to Disneyland - and as you say it is only something that usually gets traction in the minds of the rejected and the criminal, but it is enough to tip some people over the edge and into idiocy, as we saw on Wednesday.
(edit) Of course, the irony is that its Wahhabi intolerance that mitigates against the success of revivalist Islamic movements - for instance, if IS hadn't started genociding Shias, Yazidis, and Christians then they would probably be in Baghdad now.