As much as we are disgusted with ISIS's actions and from our point of view is a public relations disaster, in the muslim world it is a different story. By being the "purist" islamic group, they gain a lot of street cred. As for the Saudi royal family, ISIS can correctly point out that islam does not allow monarchies. It allows for a caliphate, and that is it. The royal family doesn't have much of a theological leg to stand on, against a group that has claimed the caliphate.
ISIS also is gaining ground in that the Saudis are seen as old and paralyzed, and the willingnes to act is attracting the youth to ISIS.
Also, ISIS is an apocolyptic group. In islam, you don't just wait around for the apocolypse to happen, it requires action on the part of muslims. ISIS members are fully expecting they can bring about the return of the Mahdi and Jesus. And part of the apocolypse is the prediction that Mecca and the Kaaba will be destroyed by muslims. Note that ISIS has threatened to destroy the Kaaba.
Finally, note ISIS's benefactors. There are three regional powers in the muslim middle east. Turkey, Iran, and Saudi Arabia. Turkey is funding ISIS. By doing so they may be able to knock out Saudi Arabia.
Just like Ahmadinejad thought he was appointed to do....he even built a highway for the Mahdi! It is interesting that everything in the Islamic Apocalyptic literature is a knockoff/counterfeit of what is found in Christianity....which could very well mean Muslims will usher in the anti-Christ believing him to be their Mahadi. Though we don't know how he'll be actually presented to the world....Muslims will definitely accept and support the anti-christ.