Politicians from both parties have lined up on the opposite side, in favor of the treaty, because it makes more sense politically than it does militarily. The answer is that he knew that gassing Americans would lead to Americans gassing far more Germans. We had chemical weapons and he knew it. That is why Hitler didn't use his. What gets complicated is trying to evade the obvious, in the interest of scoring political points by being on the side of the angels and denouncing chemical weapons.
All weapons are harmful.
That is what makes them weapons. It is short-sighted grandstanding for politicians to pretend to be banning weapons that they cannot in fact ban. But that is what makes them politicians. The joke in all sorts of disarmament treaties throughout this century has been that peaceful nations tend to abide by them while aggressors violate them. That is how Britain got suckered in its naval treaty with Nazi Germany in the 1930s and how both Britain and the United States got suckered in their naval treaty with Japan before that.