We shouldn't forget that inside many leftists is the urge to dominate others. While some on the left may have legitimate feelings for improving the lives of others, it is my opinion that many leftists are simply evil people who wish to inflict their evil ideas and themselves on their fellow humans.
When the Bolsheviks took over in Russia, they made things infinitely worse for the average person than whatever the Czars (who were hardly angels) did. Stalin wasn't an aberration of leftism. He was simply a more extreme form of Lenin who was himself an autocratic murderer.
So no matter how well society functions, as long as there are leftists we will have disruptions and leftists (antifa, BLM, Cair) trying to impose their idea of utopia on everybody else.
And as we know the leftists' idea of utopia is really hell on earth.