Saudi Arabia and Iran as countries are strategic rivals over who controls the Persian Gulf. The Wahabbist/Salafist types of Saudi Arabia (etc.) and the Shiites of Iran (and Iraq) are blood enemies when it comes to religion.
These days, the Wahbabists / Salafists (mostly Saudi initiated, but sometimes Turkey or Qatar) are strong allies of Israeli hardliners and US evangelicals (despite all the terrorism of ISIS and Al Qaida), against a looser alliance of Orthodox & Catholic Christian Arabs, as well as Shiite Muslims (and to some degree Russia and a lesser degree China).
The Middle East is a strange and harsh place, for of layer after layer of intrigue & deadly grudges. The best thing any western power can do is stay the fuck out of there, and not end up being used as dumb muscle for all the sneaky manipulators behind the scenes.
As Smokescreend said, there is zero prospect of installing a liberal democracy there; nothing installed from outside can survive and nothing genuinely homegrown would resemble the western kind. If western leaders (like Trump, or Obama before him, or Bush & Blair before that) are talking about democracy and human rights, that is just a smokescreen for the western public to be blinded by, as they gear up for starting yet another war that will spill a lot of blood and bankrupt our countries as well as theirs.