what would ISIS (ISIL, the IS) do if, for no reason, allies bombed Mecca into a 1/2 mile hole in the ground?

Mecca is in Saudi Arabia - supposedly an ally of the US. They have their own axes to grind in the ongoing wars of Sunni Muslims vs Shiites, and like the US, they are supporting various insurgent groups in Syria and would like to see Assad overthrown - it is NOT simple. For the US to bomb Mecca makes no sense as it would be an act of war against a prominent ally. Because Mecca is holy to Muslims everywhere this would also serve to unite all of them against the countries responsible. You must understand that ISIS is an artificial organization created under some of Saddam's former generals, and secretly supported by the US and Turkey. The US was only pretending to fight ISIS because they were using it as a tool against Assad in Syria, and also to generate a bogeyman to create fear in western nations - the US especially - so that the public would continue to support the governments destruction of civil rights at home. Meanwhile the US was giving them money and weapons. Then Vladimir Putin threw a wrench in their plans by REALLY effectively attacking ISIS and destroying a lot of their facilities, and exposing their secret oil trade with Turkey. The US command was freaked, and was begging the Russians for warning before attacks so that they could pull out their CIA operatives supporting ISIS. No wonder that the press is bad-mouthing Putin! So anyhow, bombing Mecca would not directly affect ISIS, but would result in Muslims everywhere attacking western countries and their possessions world-wide - not something in the best interests of the US - or anyone.

Likely become the undisputed leader of the Muslim world.

Why not just go kill the puppies of everyone in the world as well? I'm sure that will work out.

I think you and ISIS share a wet dream.

To bomb Mecca and validate the Jihadi claim that America's 'war on terror' is a lie, and in truth it's a war against Islam, would be viewed through their eyes as a gift from Allah.

"can nothing stop these madmen, so vicious and so cruel?"

A sense of oppression is their oxygen. It's no coincidence that these groups sprout up wherever there is a Muslim population that feels oppressed by a non-Muslim/secular force (eg, Afghanistan under Soviet invasion (Al-Qaeda), Shah's American puppet regime in Iran (Islamic Revolution of 1979) occupied Palestine (Hamas), occupied South Lebanon (Hezbollah), occupied Iraq (ISIS), neglected Sinai (ISIS in Sinai), etc.). America knows this, why we pulled troops out of Saudi Arabia less than two years after 9/11. Our economic interest in the region prevents a complete disengagement.

Use it as propaganda to get more recruits, a LOT MORE recruits.

Why don't you ask them.