So, let me get this straight - Assad's forces bombed a rebel chemical weapons depot.  When that blew up, poisoning the people around with sarin gas, he then bombed the hospital where the victims went, which, coincidentally, caused yet another sarin gas leak?  Why the diabolical genius of those rebels, keeping sarin gas everywhere!

Doesn't it seem more plausible that, with ISIS on the ropes, Assad was seeking to eliminate another faction that might insist on having a role in the post-ISIS governance of the country?

As for illegality, it was legal under domestic law, as the President made the necessary War Powers Resolution notification to Congress and finding that the national security interests of the US were at stake.  Under international law, it's definitely more questionable, but we have the Kosovo and Libya responsibility to protect precedents.  Roth's reasoning would have had us stand by while the Nazis gassed the Jews because they hadn't attacked us.