It doesn't make any sense. Not only was he winning the war, he had just gotten the US on his side - basically - via Trump, who generally seems to support Russian actions in the area. Not to mention Assad had supposedly given up his chemical weapons stockpiles to the UN a while back. Now, suddenly, he's got more chemical weapons and for some inexplicable reason he's dropping them on his civilians again. Given what we know, it doesn't make any damn sense. Maybe something was happening behind the scenes that would explain this sudden shift in policy, but nothing that is public helps explain why Assad would do something so stupid.