The Mullahs never claimed that it was a democracy, it was always going to be a Theocracy. The Shah was a far bigger butcher than Assad and he was bought into power by yet another CIA/MI6 coup. The CIA even set up and trained the Shahs notorious SAVAK (Secret Police) that were responsible for the torture and murder of thousands of Iranians.
The Iranians were glad to see the back of him but at least the choice was theirs. The US let it happen because they thought that the mild mannered, religious Khomeini would be feeding out of their hands but the Iranian people never forgave the US for destroying the democracy of Mossadegh in the 50’s and then supporting 28 years of extreme suffering under The Shah.
I lived in Iran for over four years post revolution and in 1990, we were some of the first westerners to go back into Iran after the Revolution and I can tell you that there were far more freedoms than any of the other ME countries I worked in. The rules are strict, about that there is no doubt, but I always found the place to be very safe and never felt we were under any threat from the people or the Regime ..... provided we followed the rules. But doesn’t that apply anywhere?
The same will happen in Syria as happened to Iran in the 50's if the west gets its way and puts another puppet government in the driving seat. Assad has to stay until the people make the choice to replace him. First stop the civil war and, as Assad has already offered, give an amnesty to all the FSA fighters. Not a surrender but a recognition of their grievances and a step toward an end to hostilities. They can then solve the problem diplomatically ..... like they should have done in the first place and would have done had the west not poked their nose in.
Then concentrate on destroying the IS fighters of Daesh with outside help at Syria's request.
One thing is certain ..... no amount of weaponry or boots on the ground will kill Daesh! It is an ideology and you cannot destroy an ideology with bombs and bullets! The only thing you will ever achieve is to reduce those willing to die for the cause and that will be through hearts and minds because nothing else will work. Reduce the support these Islamist get and they will become a depleted force, still dangerous but incapable of raising an army.