Any large rebel group can easily take 284 people hostage, this doesn't mean they are taking over. They are actually getting pushed out of most countries. For instance they are losing the fight in Mosul, one of the last major ISIS stronghold in Iraq. When the US Coalition, Kurdish forces, Russian Army, and Iraqi Army take Mosul they will be weakened.
While I believe ISIS has the firepower and resources to take over the entire Middle-East, I believe various world powers would intervene before they could accomplish the task. What would happen instead is the start of WWIII after leaders differ on opinions depending on if the US and Russia agree or not.
First of all, "the Middle East govement" is not a thing. There are many counties that make up the Middle East, each with varying forms of goverment. Secondly ISIS has no interest in being a governing body, but they would like to bend governments to their will, something they have done a great job of in the United States.
No, ISIS is not going to successfully take over the middle-east Government because they are fading quickly. The only reason they are executing people is to prove they have some power left. If they were actually rising up the ranks and succeeding, they would be more secretive to plan a bigger crime.
ISIS isn't very organized, even if it is radical, and to have a governmental takeover, it would need more structure. Even if it's a coup, ISIS would need more people, more funding, and more weapons to take over and hold more than individual territories. ISIS is clearly a problem, and needs to be dealt with.