Rather it was the people using the new system to settle old scores or for personal advancement.
Lenin, Mao, et al look at how many people had gotten into the party entirely for the purpose of abusing their positions for personal gain.
How was that allowed to happen? Did they build a system of oppression that was ripe for takeover by petty tyrants, some of whom became actual, fully fledged tyrants, whilst simultaneously shutting down the mechanisms by which workers could have power over their own lives?
This isn't about whether Stalin personally gets into heaven, plus the absurd strawman that people think he did anything personally shows a complete lack of systemic thinking, which was ironically one of Marx's great contributions to political thought. It is about whether the systems we build are liberatory or oppressive.