The current situation with Saudi Arabia and Iran appears to be a religious conflict, but actually this is all about the oil price. Obama allowed US shale oil to flood the market in order to cripple the Russian economy - didn't quite pull it off - to which the Saudis responded with more production in the belief they could put the US shale production out of business. They couldn't, and now the Saudis are in hock up to the eyeballs with their currency on the verge of collapse. So the fact Iran can also now contribute significant amounts of oil to the world market and force the price of crude even lower is seriously bad news for them. They've executed a man in order to fuel conflict with Iran in the hope they can provoke them to cross the line and end up being isolated from the global market once more.
On a day when the Chinese stock market cratered the price of crude has gone up, solely because of the Saudi/Iran situation and fears over supply.
If Saudi Arabia collapses and can no longer defend itself, IS has significant support among ordinary Saudis.