Communism, and any other utopian collectivist dream are atheistic/humanist religions. They depends on the assumption that people are inherently good and that they only do evil as a result of external factors. So if a group is evil, it must be the fault of those who are resisting participation in the collective and prioritizing components of individual liberty over the perceived "common good". It's a core tenet of their ideology and they cannot allow it to be challenged. If they accept that people are scumbags and that at least a portion of those people will act on it in the absence of external control or consequence, then they have to accept that man is a fallen being in need of an external savior.