This is one possible explanation to some of what is happening in Syria: With the Assad regime in place you have a nice little belt stretching from the Mediterranean sea to Pakistan. From the Hisbollah in Lebanon, through Syria, through Shia Iraq, and all the way to Iran.
Remember how the last war between Hisbollah and Israel panned out? It was supposed to be a cakewalk for Israel, but it ended in a draw. The West were understandable not interested in a rematch against a hugely motivated Hisbollah with supply lines stretching to the Pakistani border, and possible beyond that border as the powerful intelligence service ISI in nuclear Pakistan, closely linked to the Taliban in Afghanistan, is hardly a trusted ally.
The West wanted to break up that belt, and Assad was seen as the weak link of the chain. Hence the apparently absurd confirmed American support of groups like Al-Nusra (Al Qaeda in Syria), less than fifteen years after 9/11.
The Russians got involved of course not to fight terrorism, but to counter the US tactics of helping the 'moderate' groups like Al Nusra take control of Western Syria, including the Latakia region which hosts Russia's only port on the Med. When Al Nusra and other groups got too close for comfort, Putin draw a line in the sand. Russia's efforts have pushed the enemy forces back from the Western provinces and secured a buffer around Latakia and the port. Mission accomplished. For now anyway.
Now, the Saudis involvement and the prize of oil. The cold war ended after the US got the Saudis to overproduce resulting in a collapse in the price of oil to below $10 in the late 80s. The Soviet economy, already weakened from the arms race and the Afghanistan engagement, broke its back. The powers that be decided that was such a fine trick they wanted a dacapo.
So, the Saudis are told to overproduce oil in order to hurt Russia. In return they are given a free hand in their struggle against Iran to be the Middle East top dog. Which of course is seen to be in the West's interest as well. It's easy to forget that the Saudis are currently more than a year into a proxy war with Iran in Yemen, as the media almost totally neglects to report from it. And nothing would please the Western powers more than to see Saudi boots on the ground in Syria.
The Western powers may or may not underestimate the future problems regarding a Saudi-dominated Middle East, but they've obviously considered the options and have decided it's in their interest. It's funny/tragic to see how we're supposed to think the actions in countries like Syria, Iraq and Libya are guided by moral principles. These countries were, for all their obvious flaws, the most secular states in the region, while our closest ally Saudi of course are the exporters of that wonderful Wahhabi doctrine.
I feel for all the Syrians who are paying the ultimate prize for this geopolitical maneuvering. It's hard to see an end to it all. Making things worse must be the feeling of being locked in a conflict that doesn't make sense, with all the conflicting parties and the blurry lines of interest. In that regard I think the understanding of the bigger geopolitical picture may be helpful.
Disclaimer: This is not meant to advocate the "blame the evil West/US/Israel for all that is wrong in the world". I think the decisions are made based on genuine concerns related to realpolitik. If you or I were in their shoes we might well have feel forced to make the same moves. Ultimately these power games are a direct and inevitable function of a certain way of seeing ourselves and the world we live in. Before these views are upgraded tragedies like the one in Syria will continue to unfold. Peace.