I was going to make the arguement that the lines between left and right are blurred, and that his policies at the time were quite right, especially by todays standards, but someone already attempted to say I should read a book
As mentioned he reinforced the values that the tsars already set up, and then collectively made it way worse - hense the repression and famines.
'Waiting for Hitler' does go into Stalin's views on both of those movements, obviously more in the 1930s, he states that nazism and fascism are a form of reactionary politics that wanted to maintain capitalist society but as you mentioned he saw it as a useful thing that would destabalise the West and his other rivals.
I think he underestimated Hitler's threat and as you mentioned created a pact with him to avoid conflict while Stalin could get more land in Eastern Europe, but even though he created that pact he saw it as dangerous, and you could argue it's essentially what Putin is doing today, in that he allows it to fester (and even Influence it - there's a thought that Elon and Trump are good friends of Putin) - because by effect, it will destabalise the countries it takes foot in and cause internal conflict which means he has to do less to then take advantage of those countries.
Back to Stalin though, I think like Kotkin mentions his attitudes towards those movements were shaped by his pragmatism and it essentially betrayed his stated ideology, I think the 1941 invasion showed that his foreign policy was largely influenced by strategic considerations rather than his actual ideology.
I've heard about The Mitrokhin Archive, for those that don't know Mitrokhin was a KGB archivist who initally offered these notes to the US - who said they were fake - and then offered them to MI6 instead.