Generally speaking, for any kind of "-ism", the suffix "-ist" produces the noun form and "-istic" produces the adjective form. But there are some "-ist" suffixes that are acceptable or even more commonly used as an adjective. Examples include fascist, humanist, racist, socialist, absurdist, etc.
Why do these adjectives not take the usual form, i.e. fascistic, humanistic, racistic, socialistic, absurdistic?