I can comment on it.
ISIS, the Islamic State, is an enemy of Assad because for ISIS, the Assad regime represents an infidel government, the Alawites, which is an offshoot of the Shiites. For ISIS they should be executed as apostates, and particularly, Syria is called al-Sham. Al-Sham means the old area. Syria, Jordan, Israel, and Lebanon are part of the prophecy of Muhammad. This would be the first area from which the jihad would start to expand, and that's why they are focusing on Syria.
ISIL, the Islamic State in Syria and the Levant, was established first of all in Iraq and then in Syria in order to organize and unite all Islamic forces under one flag of Islam to establish a very strong basis, a stronghold inside Syria, to be used as a basis for launching the jihad.
There is no way for compromise between the Islamic State and the Assad regime. There is no way. They are not talking about compromise. They aren't looking for any negotiation with the Assad regime. He is a prime enemy. If you have listened to what Abu Bakr al-Baghdadi, the caliph, the leader of the Islamic State, has said, it's very important. He talked about the next target, which means Saudi Arabia and Jordan. But he said to the followers of the Islamic State that there is a religious obligation to kill the disbelievers. This is the priority. But they should also remember that their first priority, their first enemy, is the Shiites. They shouldn't misunderstand him. They also have to look after the Shiites. They should not give the Shiites control of the area on a silver platter.
That explains in a nutshell the animosity of the Islamic State towards Iran and Bashar al-Assad of Syria.