Your framing is only possible in hindsight.
No, plenty of people at the time condemned Lenin as a hypocrite without any real principles and a tyrant-in-waiting.
if he wasn’t a true believer he could’ve done literally anything with his life. Painting evil mass murderers as fakes is appealing, but in this case it doesn’t stand up to the slightest scrutiny; both had already been committed radicals for way too long by the time of the revolution.
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?