Human rights are a bourgeois concept. They were thought up during the Enlightenment when a bunch of upper-middle class/upper class philosophers decided that that they thought was universal and right, just like the laws in the feudal era and the slave empire era.

There are many things for leftists to criticize Stalin for, but human rights violations should not be one, as that would be like criticizing Mao's China for having a one party state as "anti-democratic".