They suffer from a psychological disorder known as the Thanatos Complex.
I am in the middle of a Thomas Sowell book “Intellectuals and Society”that you may find interesting. While I’m not sure that he expounds on the specific history or some watershed where this all began, he does opine that if they were on the right (or perhaps more accurately non-interventionist) things would get on quite fine without them. Being on the left however allows them to come up with their own ideas and solutions and ultimately pat themselves on the back for showing everybody how smart they think they are.(I am paraphrasing significantly)
If you will permit me to ramble, back to my Thanatos comment. It seems to me that the left under the rather specious guise of equality or fairness is seemingly bent on their own destruction and the demise of all mankind. You can find this everywhere from their staunch position on abortion. The environmental movement demonizing man as the sole factor for everything negative under the exosphere. Marx himself was the bourgeoisie that he sought to eradicate. I am absolutely astounded by the lefts hatred of the free market, a system where the workers of the world can set their own wages and prices based on their own actions. The equality they want can only be achieved by the deceased.