No, plenty of people at the time condemned Lenin as a hypocrite without any real principles and a tyrant-in-waiting.
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. Nobody goes to prison for principles they don't have.
Are there any major mass murderers who came to power, rather than inheriting it (literally or effectively) who weren't true believers by your criteria?
I mean any mass murderer has to be a true believer in something, at least so they can sleep at night; 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. 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.