It seems to me that almost any adjective can be used as a pejorative term, that it is all a matter of context.

Adjectival nouns share this quality, and the xxx-ist form seems almost made for the expression of dislike. It obviously has other uses, but there's something about the shape; try saying 'Organist' aloud a few times, it soon sounds like derision or contempt.

As for your specific dislike, well a purist is often only a hairsbreadth away from a fundamentalist, which is in its turn only a cockstride away from an extremist, and so onwards. The term may sometimes be appropriate, as may the actual attitude, but I feel that both should be used with restraint and due forethought.