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Why が? What exactly does くそが mean because it seems to mean something different from くそ. And I can't think of anything the might be omitted from the utterance.
We use this word not only in くそが but also in phrases like 「馬鹿が、アホが、お調子者が」, right after the swearwords.
〜が is used for stronger resentment or malignance or just to emphasize.
Grammatically, it is classified as a sentence-ending particle (終助詞) in Japanese.