So you want Sharia A and IS wants Sharia B and JAN may want Sharia C. Everyone will say their Sharia is the best. I can't really fault IS for wanting to establish their own kind of Sharia anymore than I can for JAN and FSA. They are all the same to me. Meanwhile, the people who don't want to be ruled by religious dogma are fighting for Assad.
You see things from Western portal, in the Middle East there is no religious dogma as much as media may try making it seem. It has always been liberal dogma for the past decades, at least in the Arab world. Arab regimes hardly impose Islam, the only monarchy that does so is Saudi Arabia. Because Saudi Arabia is historically significant to Muslims and is birthplace of Islam, its laws, interpretations, etc....Even the ones that do, you will still see secular lifestyle in them because they only do so for credentials...
In the Arab world it is a struggle against Arab liberal/Shia/Israeli/Western dogma. The struggle is led by Sunni Islamists. Shia Islamists or Iran don't share the same struggle, they tolerate and embrace secular lifestyle(another evidence no way they qualify for position/glory they assign to themselves) and only work to achieve Iranian interests and don't share sentiments of majority. Shia Islamists are some of the worst because they insist that their Persian cult ideology is superior to Islam. Many Shias are normal , good people. But there are boundaries where can say this ideology doesn't follow Islam and serves a foreign ancient empire. No Sunni's consider it to be legitimate. It is dangerous because this minority has loyalty to Iran but most importantly they live on another Universe where their teachings are legitimate. Shia's mostly don't actually believe in Islam, but they like the ideology as it is perfect tool for Iran to try gaining influence in the Arab world. This is why Shia's are an obstacle, and if you're an obstacle to ending Muslim suffering/persecution than in our religion you are to be fought. This is self defense, as much as our enemies try portraying it as aggression. We are not the ones forcing ourselves on others. Shias/Arab dictatorial liberals/West/Jewish people are forcing theirselves unto the Middle East and causing misery for the Sunni population.
The case with the West is commonly known. But to add point, contrary to what Westerners are told, ME Islamists want nothing to do with the West. They just want to be left alone and remove the oppression against their peoples in the region. There is no problem with middle easterner Islamists and West being at peace and entering agreements. However this isn't possible when the West is propping up so called 'liberals' to target islamists and militarily targeting Islamists on their own. Whilst shias/Jews also do the same thing.
So yeah, maybe some people in the ME are fine with being subgjuagted to oppression because they either happen to be careless wealthy liberals or Shia's(who have different ideology and pursue their own interests separately from the main body of Muslims, which is why they're so disliked), but those people don't matter. And the more they increase the suffering of the people the sooner there will be an all out regional struggle against them until they end their conspiracy/attacks/subjugation/siege/dominance of Muslims. And this struggle is the most just struggle on the planet earth to take place with exception of struggles of prophets of God. Sunni Islamists are taking up a holy and just cause and no doubt will be opposed by the majority of the world, but numbers don't matter. Being right matters, and they are right, period. The only ones who don't think so are the misdoers, shameless and corrupt people.