If it was Syria/Russia then 3 possible reasons:
1. a rogue General without Assad's knowledge;
2. Assad got too confident that Trump wouldn't do anything (which still doesn't explain why he would have done it);
3. Russia/Syria did it to deflect attention from the Trump/Russia investigation (which could explain Trump's bizarre tweet about making peace with Russia soon).
The bio-medical samples collected from three victims during their autopsy were analysed at two OPCW designated laboratories. The results of the analysis indicate that the victims were exposed to sarin or a sarin-like substance. Bio-medical samples from seven individuals undergoing treatment at hospitals were also analysed in two other OPCW designated laboratories. Similarly, the results of these analyses indicate exposure to Sarin or a Sarin like substance.
Director-General Üzümcü stated clearly: “The results of these analyses from four OPCW designated laboratories indicate exposure to Sarin or a Sarin like substance. While further details of the laboratory analyses will follow, the analytical results already obtained are incontrovertible.”