"the left"? You have to be joking! Even US cold war-era anti-communist propaganda wasn't lying this blatantly. Mass incarceration, execution, etc. are a "skill" of all totalitarian regimes, no matter what social/economic policy they happen to support. The church in the middle ages, kings in the era of absolutism, the Axis powers during WWII, regimes of warlords past and present, the various religious states that are doing the bulk of it today... all of these regimes censored, imprisoned and executed their opponents and few could be considered "left".
The original comment was entirely arbitrarily limited to the 1900s, and as it brought "the left" into the discussion unprompted, I can only assume this was done intentionally to justify specifically going after left regimes. So firstly, I don't see why a counter-argument should stick to this arbitrary time frame, and secondly, some of my examples do in fact apply to this time frame too.
