When Israel unintentionally killed civilians, lol!!Assad is only marginally better than ISIS. The dictator's false claims of secularism aside, the only thing that really separates the regime from ISIS, and thus causes many in the West to support the former, is that Assad speaks English, is well-groomed and wears a suit, while ISIS is a bunch of screaming bearded fanatics. From any moral standpoint, however, they aren't much different. Under ISIS, the will of the state is enforced by the roaming Hizbuh religious police, and those who violate ISIS' version of Islam or dissent are punished via death and torture. Under Assad, the will of the state is enforced by Shabiha thugs and the mukhabarat, and those who dissent against the regime are punished via death and torture - it's been documented by the UN, human rights groups, and defectors (via photographs) that the regime has tortured perhaps thousands to death. Both groups use indiscriminate warfare against their enemies, and it's a safe bet that Assad has killed more civilians than ISIS has. Both groups have committed sectarian violence, with the Shabiha and NDF targeting Sunnis and ISIS targeting non-Sunnis.
Assad also happened to support al-Qaeda in Iraq when we were there, and only now is his cavalier attitude towards the jihadist movement coming to bite him in the ass. His brutal occupation of Lebanon happened to go unnoticed by almost everyone who took the time to cry foul when Israel unintentionally killed civilians in its war against Hezbollah.
Finally, "Assad vs. ISIS" is the definition of a false dilemma. Why should we have to choose between a fascist dictatorship and a Wahhabi caliphate when there are plenty of other options available? While it's improbable that any Syrian rebel faction is secular by Western standards, not every Islamist in Syria is of the same caliber as ISIS or even al-Nusra. The YPG Kurds are both secular and democratic (Assad is neither).