Most Negative Western action of the Cold War?

[The Mikado]

It ends up varying quite a bit depending on your definition of action.

I mean, installing Pinochet is a US action (with cooperation from Pinochet himself, of course).  Pinochet's activities as dictator of Chile are not US actions.  Similarly, the US bombing of Cambodia and the overthrow of the king helped lead to the rise of the Khmer Rouge, but the Khmer Rouge's massacres were its own actions.  Defining Pinochet's atrocities as de facto American atrocities really takes him out of the picture and contributes to the incorporation of Latin American history into the US history narrative by removing Latin American agency and is a gross misuse of history as a discipline IMO.

[The Mikado]

General Marshall was a great alarmist who is chiefly responsible for the fall of Nationalist China.

I had a post upthread about people eliminating agency of foreign peoples and annexing their history into US history, and this is a great example.  Nationalist China wasn't "lost" by anyone in the US, it was lost by the incompetent, brutal, and corrupt Chiang regime which managed to alienate vast swathes of the population and proved utterly unable to defeat the CCP.  Chiang lost the Chinese Civil War (or rather Mao won it), not the US.