I mean, it’s not an absolute, but Wikipedia defines Left-wing politics as “the range of political ideologies that support and seek to achieve social equality and egalitarianism, often in opposition to social hierarchy as a whole or certain social hierarchies”.
Stalin actively repressed and killed ethnic minorities during the Great Purge. That’s absolutely not egalitarianism. I don’t know much of his politics but if he was trying to be a communist, his government was not really a “Dictatorship of the proletariat”. He could’ve written anything, actions speak clearer than words.