You can certainly be explicit about the negative evaluation, e.g. "racism sucks", "being a racist is not cool". The issue with language is that context imbues words with meaning. So when you use a term in such a way that the dynamics between the term and the context leads to associating the negative connotation of the context to the word itself, further usages of the word then carries that negative connotation with it. In the case of slurs, they no longer (or never did) carry any neutral meaning. So the difference between expressing distaste and using a slur is how the negative connotation is expressed.