Carmen isn't your typical antagonist because we don't know the extent of the harm she has caused until the very last pages of the novel. For the majority of the story, Carmen appears as an immature child with an annoying giggle and persistent need to flirt with every man she meets. She's a nuisance mostly, but not really a threat.
Or at least she isn't until the closing pages of the novel when she attempts to kill our main man. As Marlowe finally pieces together all the clues and discovers that Carmen murdered Rusty, we realize that she has been the hidden antagonist the whole time. Her child-like innocence turns out to be all an act, and she comes to represent the kind of moral corruption that Marlowe hates: the covering up of the truth in the belief that you can get away with murder.