Which is just the sort of intra-socialist mudslinging that would’ve been a dime a dozen in the decades leading up to the revolution.
Okay, but it clearly demonstrates that the framing of Lenin as the fascist scumsucking mass-murderer-in-waiting that he was does not require hindsight, as numerous socialists at the time, by your own admission, recognized that fact.
Nobody goes to prison for principles they don’t have.
That is an incredibly naive position to take. People willingly suffer for all sorts of fucking reasons, and principles are pretty far down the list.
I mean any mass murderer has to be a true believer in something,
... "their grip on power" comes to mind.
at least so they can sleep at night;
... I don't even know where to begin on this one.
the question is whether they’re true believers in the thing they’re espousing in public, and in that case my example would be Hitler. He believed in the bigotry and genocide to be sure, but the “socialism” part of “national socialism” was obviously bogus.
He also didn't espouse socialism in public.
That said, most mass murderers do at least vaguely believe in the reasons they espouse for the mass murder; it’s hard to be a mass murderer otherwise.
So your answer to the question is 'no'.
Chiang Kai-Shek? A true believer (in what, God only fucking knows). Pierre Laval? A true believer. Jean-Bédel Bokassa? A true believer. Blaise Compaoré? A true believer.