I have a „theory“, which I call „Hooligan theory“:

You can smuggle hooligans into a football stadium, bypassing the security personell or bribing them. You may think this is a good idea in order to counter-balance the guest teams hooligans or to contain them. But, once you got 5000 hooligans into an arena with even 80000 other spectators there is no way you can control those guys. It´s an illusion to think they listen to you or that you can predict or even determine their actions.
Eventually there is a good chance „your“ hooligans vandalize your own assets and harm your own team and fans.

What has this to do with Syria?

The Nato security council member states or „FUKUS“ (France/UK/USA) along with their regional allies Turkey and the Gulf states Saudi Arabia and Qatar decided that the ultimate goal of removing the pro Iranian Syrian State led by Bashar al Assad justifies literally each and every means.
Thus, everyone who volunteered to fight the Syrian government, no matter whether Syrian or not, no matter whether secular or radical Islamist, no matter whether criminal or „clean“ was supported directly and indirectly with arms, money, military training, intelligence, equipment, medical care, etc.
The myth of the „moderate“ opposition, supposedly fighting to establish a liberal, democratic, human rights abiding, „pro western“ state is long busted. The broad majority of „Syrian“ rebels are sectarian and radical Islamists. Many of them are not even Syrians, like the ethnically Turk (or Turkic) „Turkistan Islamic Party“ or „the Chechen Jaish al Muhajireen wal Ansar, the Moroccan Harakat Sham al Islam“.
http://www.longwarjournal.org/archives/2015/04/turkistan-islamic-party-had-significant-role-recent-idlib-offensive.php

It is extremely delusional and childish to assume that tens of thousands of well-armed and battle-hardened Jihadists who have gotten accustomed to roaming their (and other people´s) country to kill „infidels“, „apostates“, „traitors“ (e.g. fellow Sunnis who fight in the Syrian army) or simply „Shabiha“ (a derogatory expression used to defame and dehumanize all kind of Sunni and non-Sunni militias and civilians who reject the rebels) would lay down their weapons and re-enter their ordinary civilian life on the day the Syrian government falls and Assad is killed.
This is pure nonsense. We have seen how this did NOT happen in Afghanistan after the Mujahedeen first defeated the Russian army and later the communist government of Najibullah.
We have seen what happened and is going on today in Libya, almost 4 years after the „revolutionaries“ liberated that country and killed Ghaddafi.

Afghanistan is an interesting case. The Russian army and it´s airforce did not kill as much people in 9 years as the Mujahedeen did in Kabul and some other cities through their daily shelling with mortars and artillery. Naive people argue that Assad must be removed to stop the killing, the barrel bombs etc. Look at Afghanistan. There, the distinction was not Shia bs Sunni but Pushtoons vs. Non-Pushtoons, but after the departure of the Russian everybody allied with everyone and fought against everyone else. There was cross-ethnic fighting as well as inter-ethnic (Pushtoon against Pushtoon) and „inter-sect“ (e.g. Sunni Tajiks against Sunni Pushtoons). The communist Uzbek commander allied with the mainly Tajik Rabbani government but earlier also with the Taliban. There was hardly a greater mess than the Afghan civil war which has not really ended to this date.
There is little reason to think something similar could not happen in Syria. Radicalized and violent rural based rebels, often uneducated and unemployed, but indoctrinated with hateful sectarian ideology preached by the Wahhabi Sheikhs and Imams, whose sermons are broadcasted on Arabic satellite channels are out to kill all those „Shia“ heretics and non-aligned pro-government Sunnis in what they consider „revenge“, although many of them coming from distant places such as Chechnya and central Asia, Northern Africa, the Gulf states, South East Asia or Europe definitely never suffered torture or any other repression in Syria which they did not know prior to their arrival for „Jihad“.

In all likelihood these „hooligans“ won´t stop „Jihad“. Rather they will export Jihad to the next place where they assume „infidels“, to the next „dar al harb“, for example to Lebanon, where they will declare war to the Shias who – although a minority – constitute the single biggest community in that country. These people are not „freedom fighters“ seeking to build a secular state. None of them gives a damn for any pro-western „Syrian National Council“ (SNC) living in Istanbul or elsewhere in comfortable safety. To pretend that any foreign-based Syrian expatriates wearing ties under their clean-shaven faces represent the myriad of militias fighting against Assad (and often against themselves) is to fool unknowing people. The militias are the hooligans those SNC people pretend to exert control over while the former not even take notice of them.