I find it hard to believe Assad would be this dumb. Secondly, it was never proven that he used them in 2012. It was very likely a false flag then and could be now.
The US Government has murdered vast numbers of civilians in places all over the world. While not using chemical weapons, one could make the same argument used by Trump with his aggression against Syria. That is, it is a war crime to murder civilians no matter the weapons employed.
The plane that dispersed the chemical weapons in the attack flew out of this base and returned to this base - Assad's base.
Assad's response to the strike was NOT, 'Hey, we didn't do it' - it was BS about the destruction of the base harming his ability to fight ISIS.
Assad was caught dead-to-rights on this one...and Putin and Russia can STFU because THEY have been busted as either COMPLICIT or INCOMPETNT! It was their responsibility to ensure Assad had no more chemical weapons - they swore he had no more. They lied....or (as they might claim) Assad duped them, making them look like idiots who are defending a War Criminal.