He's a typical tenured professor in a liberal college. And, he might have an argument: according to Soviet law, all of the trials and executions and prison terms were legal and just, with signatures of the politburo members affixed to each and every order of execution. There was no "crime" insofar as the courts were concerned; how could there be?
However, anyone who's surprised by his stuborn, pig-headed, defiant, demagogic, and expletive-laced rebuke must never have attended a college history course.
As a history majour, I can tell you this guy is an archtype. Most of them seem to have severe personal and mental problems, and have no shame in projecting their weirdness onto the classroom.
For example, my first class in Russian Revolutionary History was all about how Abraham Lincoln was Adolf Hitler's Role Model... and it went down hill from there.
"Cowards & idiots can come along for the ride but they gotta sit in the back seat!"