I don't consider leaders that gass their own people "the lesser evil".
At some point the situation has deteriorated so much you can't just reinstate the previous status quo. Let's not forget that Assads regime is a big reason the situation is what is today.
The country needs stability, but Assad is not what will bring it long term.
I wasn't noticing a migrant crisis when Assad and Gadafi were in power, even though the populations were relatively free to travel around. I'm not sure how you can say Assad's government would be worse than the shitfest that will take place among FSA factions and ISIS once he is toppled. If you hadn't noticed, Libya is going to hell, and we all know Iraq might never be whole again as a country.
Assad was one of the reasons, but Western allies with Saudi Arabia fueled the civil war and now the makeup of FSA is unrecognizable. There's huge bands of foreign insurgents, including fucking ISIS. Its not their civil war anymore. Assad is essentially in a war with fighters bred outside of his own country.
The West should either go ALL in, topple Assad, topple the extreme regiments of FSA, spend the next couple of decades establishing a functioning democracy, or they should get the fuck out and stop inciting FSA. Let them fight their own fight.