We have national interests in the middle east. Anything that serves to destabilize the middle east is against our national interests. The unrestricted use of chemical weapons is a destabilizing force in the middle east.
We have personnel in the middle east. Unrestricted use of chemical weapons in the middle east is a threat to those personnel, and is not in our national interests.
We have allies and alliances in effect in the middle east. Unrestricted use of chemical weapons poses a risk to those allies and alliances, and is not in our national interests.
The United States is a signatory to the Chemical Weapons Convention arms control treaty. It is in our national interest to see that the terms of that treaty are followed by other member states who are also signatories, such as Syria. One of the key features of the treaty is that it prohibits the use of chemical weapons.https://en.wikipedia.org/wiki/Chemical_Weapons_Convention
The use of chemical weapons in general is prohibited by international law. It is in the national interest of the United States to support that prohibition under international law.