Assad gets pressure from Eastern Orthodox organizations under Putin. If Assad wants continued Russian support, he must adhere to a secular system of govt even though Iran wants a Caliphate established across the Middle East.
The last thing that Assad wants is a Caliphate; that was the goal of the ISIS rebels. Assad is a secularist Baathist and a member of the minority Alawite religion.
Which is why I've never been all that upset about Russia's current presence in Syria. Certainly several light years better than what Obola and the Witch wanted.
Assad is a member of Alawites, an offshoot sect of Shi'ite Islam. They are a minority in Syria, if radical Sunni Islamists ever take charge of the whole country, all members of his tribe will be wiped out. So you can see why Assad has a lot of blood on his hand. He has to fight or his tribe will get eliminated.