IF Assad’s forces did this (chem attack) — the jury IS still out, I believe — the “reason” is that this is how he and his people (and much of the ME) think: They dominate by force and fear. You cannot apply your precepts & logic.
Trump has said he’s not interested in defending the Kurds, let alone in attacking Assad. Assad’s simply taking Trump at his word. I don’t often agree with McCain, but in this respect, he’s completely right.
Assad is probably figuring that while the war is still on, he’s got a pretty good excuse for slaughtering as many of the Sunni Arab troglodytes currently fighting to establish an Islamist regime in Syria as he can. Because if he loses, the Alawites will be on the receiving end of the same thing. And that’s the reason why he’s using every means at his disposal - economically, Syria is a basket case - like the Kurds, Alawite is dependent on the goodwill of foreign sponsors to survive. The day one of his sponsors ends that aid, he might as well pack up and leave on the next plane. He needs to end the war tomorrow, if possible. And given Trump’s all-clear signal, where he indicated he wanted to end any US presence or efforts in Syria, it’s understandable that Assad figured he had obtained Trump’s tacit approval for the gas attack.
I don’t disagree with most of that, in some sense, but it still “violates” civilized(?) concepts of how wars are fought. I think fear needed to rule is still the big factor: If Assad kills or drives out most everyone but the surviving Alawites and a few others (some Christians, etc.), he ends up with not much to rule. He needs most of the population alive, but cowed. Otherwise, this never ends, because he winds up with a resentful majority, with plenty of outside players with money who still want to stir up trouble, and all this IS in the ME, after all, where infighting & killing is the regional sport.
As far as “logical” goes, setting aside the in-country fear factor, Assad’s logic is further flawed: While much of the West can’t do much because of those defense cuts you mentioned, the West does recognize that used of “WMD” by 2nd and 3rd world parties just CANNOT be allowed to get out of hand. And we (the US) are still out there as the heavily armed “policeman”, like it or not. SOMEBODY has to take that role. Who else would FReepers prefer? The risk to most of the civilized world of not taking action is just too great to hide from. The Khadafys, Saddam Husseins, and Assads must at some point be shown such actions (chemical attacks, etc.) are intolerable, even if those they are unleashed upon “deserve it”.
Now, the all above IS predicated on proof Assad is to blame. I am not sure of that. Yes, that means that if a US allied group did it... we should smack them, instead. Hard.
Something else occurred to me. What if Trump’s statement about getting out of Syria was a “test” to see if Assad would do something stupid?