I think it's interesting that people are called "a racist" or "a homophobe" for example, but I've never come across "a sexist" or "an ageist". I suppose "a sexist" would be "a misogynist" or occasionally "a misandrist". It seems that some protected characteristics define a person's whole identity, or are considered a very significant part of it, and some are just incidental to a person's identity. Why is that? Is it an indication of how seriously the protected characteristics tend to be taken?
Religion/belief is interesting - there are atheists and theists, but there is a wider range of competing worldviews, not all of which are fundamentalist. Pregnancy/maternity doesn't seem to be an identity in the same way. Attitudes to disability are sometimes talked about as identities - ableist is used, but I don't think I have heard anyone described as "an ableist".
The difference between calling someone "racist" and "a racist" is subtle, but to me "a racist" is assigning an identity, something innate and irredeemable, as opposed to a racist action or comment, which suggests a one-off or intermittent attribute.
So Sandie Peggie has been tarred as "a racist" and "a homophobe" and "a transphobe" as if those things are all self-evidently true, and as if they define her whole character. We can dismiss "a racist" as a bad person, which allows us to ignore everything positive about her (can't we, Christinapple?) and to assume that she is also "a homophobe" and "a transphobe" (isn't that so, Jane Russell?). This is very lazy and reductive. I don't at all like her sharing sick jokes about a natural disaster, with racist slurs. I do not believe that this is a description of her whole character.