Here is an interesting article to understand the crisis between Saudi Arabia and Iran.
Almost all the oil fields in Saudi Arabia are under Shi'ite territories. This is much like Russian oil fields, which are mostly in the Muslim territories of Chechnya, or the Chinese mineral territories which are in the Uyghur territories of Xinjiang. The loss of those oil and mineral fields would make those regimes look broke or weak, providing the justification for them as to why they are determined to keep such territories by hook or crook. Massacre of unarmed inhabitants is thus seen as a collateral damage and nothing more!
In the words of Jon Schwarz, "If this section of eastern Saudi Arabia were to break away, the Saudi royals would just be some broke 80-year-olds with nothing left but a lot of beard dye and Viagra prescriptions." While such claims are exaggerations, the article sheds some light to understand the current crisis.