Sort:  

Arguendo assuming Assad did these crimes--which it seems claims of past attacks have been based on misinformation according to Theodore Postol--creating more territory for radical Wahabbists could lead to even more people dying in chemical attacks, so very unlikely to be worth it.

https://www.nytimes.com/2016/11/21/world/middleeast/isis-chemical-weapons-syria-iraq-mosul.html