My dad used to work as an at-large instructor teaching fire/police departments what to do in case of an attack involving chemical weapons (this was a thing they were very worried about post 9/11).
One of the concepts in the material involved "Mutually-Assured Destruction." So let's say, hypothetically, Russia attacks *checks Wikipedia* Kharkiv (est. population 1,433,886) with a WMD. Now, you have given any other country with WMD-style weapons reason to retaliate by attacking *checks again* either Yekaterinburg (1,495,066) or Kazan (1,257,341)
I'm not saying Putin's not crazy enough to do it, but he would open up a Pandora's Box where he could cause a lot of problems for himself and Russia if he did.
