Iran, with Russia's backing, wants control of the entire Middle East

ISIS is a pawn in a larger game. If Assad can bribe ISIS to become it's mercenary army against people opposing Assad then so can Iran and Russia. This is a given.

The word Assassin came from Hassassin which was a group of assassins in ancient history in the middle east. ISIS wants to survive, so it will take jobs from Iran, and Assad and likely Russia too in order to survive whatever may come. They want to be a country and likely will do anything to remain a country.

Will Russia and Iran and Assad betray ISIS?

Of course. But maybe not now until Assad is more firmly entrenched by killing off any real opposition outside of ISIS. So, ISIS like Russia is not entirely what it says it is about in this situation.

If you think of ISIS sort of like the KGB in the Cold War, both will do Anything to survive at this point. So, underestimating ISIS or Russia or Iran or even Assad is not useful at this point in history.

ISIS is a movement that armies cannot directly fight and win because they have 1.2 billion Sunnis to draw an army and financing  from ongoing.

Russia is a country that needs the price of oil to go up or Putin will be out of office within 5 to 10 years at most.

Iran has really pissed off 1.2 billion Sunnis. So, their only chance of survival now because there are only 161 million Shiites total on earth, is to develop nuclear weapons just like India, Pakistan and North Korea have. Otherwise during the next 100 years 1.2 billion or more Sunnis will rise up from the injustices and wipe them out. However, remember Russia has nukes too and is allied with Iran, and Syria.

This is what they whole world is facing right now. And who just joined Iran supporting Assad in Syria? 

Russia.

Who do 1.2 billion Sunnis want to get rid of now?

That's right. Russia.

Saudi Arabia (the richest Sunni nation) drove the Soviet Union into bankruptcy and dissolution in the late 1980s already by lowering oil prices.  What do you think they are trying to do to Russia and Putin once again?

And at the same time they are driving oil drillers and shale oil miners out of business all over the earth. How long can they last at 20 to 25 dollars a barrel for oil?

As long as it takes for them to survive and to drive Russia and all oil drillers and shale oil miners out of business but them worldwide.

And what happens then?

Any country not having switched away from oil as their main power source will go bankrupt.

(I figure we will lose about 50% of all governments on earth at that point.)

To the Saudis this is the price for their survival and for their government to survive.

For the world it will be instead a bloodbath of starving people not able to afford to eat and ending their governments in retaliation.