> why is the postmodern view that all people, including violent criminals, are victims/products of their environment, but racists are uniquely and irredeemably evil to their core?
Well, I don't agree that that is the postmodern view, unless you define "postmodern" to specifically mean that view so that it is tautologically true; particularly, I don't believe that its all that common for any person (regardless of whether the label "postmodern" applies) to hold simultaneously to both of those beliefs. They may both be beliefs that are visible in some segment of the population, and there may be some overlap, but I don't think there is much correlation.
But, in any case, even if they were commonly held together, there is nothing inconsistent in believing that all beliefs and behavior patterns are products of the interaction of the individual with the external environment, and believing that there are certain beliefs and behavior patterns that, once established, are not correctable through any change in the environment that other people can effect (even if the same behaviors/beliefs may be made less likely to manifest in those who have not already manifested them), such that those behaviors/beliefs only go away when the people holding them die.