Of course Stalin backed Antifa - Germany invaded and attempted to conquer Russia in WWI and then the Soviet Union in WWII.
What you seem to have missed in that history class is the fact that after WWII, he became increasingly paranoid about the largely Jewish anti-fascist organizations, disbanded them, and began a pogrom in which the Kremlin began arresting, trying, convicting, and executing them on false charges of treason and espionage.
As for brutalizing Americans, that's to be expected as our own society is forced to take the dark path down the alleyway of fascism by our own right wing.