Do you believe that every single action one must do be perfectly in line with leftist ideas to be a leftist? Is it not possible to commit evil for the purposes of, in their minds, securing the chances for achieving Socialism and Communism?
If an action has such a big impact and such little payoff as the ethnic cleansing in the great purge, I think that disqualifies you from actually being a leftist.
By that reasoning, even christian fundamentalists would be leftists: they want everyone to believe in god and adhere to their teachings so that we can all be at peace for eternity in heaven. But if what they’re doing to achieve it is Crusades, discrimination and antagonizing progress, the end result isn’t worth it (especially since, like with the great purge, there’s no guarantee all this suffering will actually bring results).
And we can learn from the mistakes of people even if we don’t think they align with our political stance, me considering Stalin a leftist or not doesn’t change my opinion on what he’s done.