The US has not put forward any credible evidence that Assad or his regime initiated the chemical attack. And the circumstantial evidence argues against it. All we really know it's that such weapons are in Syria, they were apparently not all destroyed. Who does the attack benefit most? Not Russia (there is nothing positive in this for them), there is mostly no good news for Assad unless he was doubting Russia and wanted to cause a clearer split with the US, but mostly he just poked a hornets nest. Trump benefits in terms of distraction from his many problems, but it is very cynical and  guys like Secretary of defense Mattis, seem to support the action, so  I doubt it was an all out cynical play, but it is possible. ISIS benefits, Al Queda benefits, some rebel groups benefit (it creates chaos which is to their benefit and causes friction between those that have a mutual interest in seeing them ended it in the case of rebels bringing pressure to bear on Assad).