At any given moment there are a million foreign people freely traveling around the US- some on vacation, some visiting family, some for school, some for work, some for medical treatment, etc. It's relatively trivial for a nefarious organization to find a person who speaks English and can legally enter the US.

For Russia (among other places) none of this is true. Visas to enter the country are harder to acquire, movement within the country on such a visa is still constrained, a smaller number of foreigners means easier for intelligence to monitor, and far fewer people have the language skills or physical characteristics to blend into the local populace.

In the first couple months of the war, there were lots of explosions/fires at major Russia industrial or infrastructure sites that got reported; I’ve seen fewer reports recently, but I don't know if it is happening less or Russia has clamped down on the news more effectively.

>refineries venting poison gas into populated areas

Hard to think of a better way to become the villain in the story. This has not occurred, but not necessarily because the means of making it happen aren't there.