Unless that information makes the statement seem extremely biased, which it does?
> During the 1900s the left killed people on a scale never seen before in human history
And during the 1900s totalitarian rightist killed people on a scale never before seen in human history. Industrial murder factories, with brutal precision, murdering thousands per day by the trainload, with an assembly line of transportation, robbing, murder through poison gas, cremation? Never has humanity sunken so low. Brutal ad-hoc genocides of people murdering another group with their hands due to hatred or misinformation or whatever is more understandable and explainable than cold blooded mechanised precision mass murder on an industrial scale.
Famines due to ethnic cleansing, stupidity, mismanagement and ignorance are aplenty. The Bengali famine is comparable to the Holodomor in scale and reasons, even if the Holodomor was probably more intentional. Nobody has done what the Nazis did at that scale.
So yes, it's extremely stupid to single out totalitarian leftist regimes for their crimes against humanity, as if it's a uniquely leftist thing. It's obvious the people doing that have an anti-left agenda, which is honestly just being stupid.