Read that 2013 BBC article, and it sounds exactly like the articles we see today in the news.
Each time this exact type of attack happens, we see the exact same rhetoric trying to drag us in to war in Syria. A move that would benefit Israel and the US, but they cannot invade without international semi-approval, or else world powers might start turning on them. They have to create the appearance of a humanitarian crisis, just like the "Weapons of mass destruction" in Iraq, in order to get the social approval room to make a move.
Both other times the Sarin Gas attacks happened in Syria, it was at a point where Assad is winning, and thus has no need to do it. Especially knowing it's a known war crime that WILL bring a response from the US.
ISIS is 99% defeated, and NOW he's putting this near-certain win in international jeopardy by making a move that could've been easily accomplished with a conventional bomb anyway?
The only motive that makes any real sense is that the west caused this event to happen, to get their foot in the door as a pretext to a US/Israel backed invasion.