There is one thing the West can do that would be unambiguously good. Also, cheap; and it wouldn't involve killing anyone.
Nerve agents like Sarin aren't black magic; they're close relatives of organophosphate insecticides. Medical treatments exist. In particular there's a gizmo called a NAAK, or Nerve Agent Antidote Kit. The drugs it relies on (neostigmine, atropine, and diazepam) are all more than fifty years old and dirt cheap; they won't save someone who has inhaled a high lethal dose, but they'll stabilize someone who's been exposed, hopefully for long enough to get them decontaminated and rush them to a hospital for long-term treatment. Mass Sarin attacks are survivable with prompt first aid and hospital support.
We should be distributing gas masks, field decontamination showers, NAAK kits, and medical resources to everyone in the conflict zones. Government, civilian, rebels, it doesn't matter. By doing so we would be providing aid that was (a) life-saving (b) cheap, and (c) put a thumb on the side of the balance in favour of whoever isn't using nerve gas. We'd also be breaking with the traditional pattern of western involvement in the region, which is to break shit and kill people, mostly innocent civilians who were trying to keep their heads down. It wouldn't fix our bloody-handed reputation, but it'd be a good start.