Rebecca, though dead, invades nearly every line of the story. Most importantly, though, she threatens the happiness of our protagonist, Mrs. de Winter. Mrs. de Winter seems to live in Rebecca's shadow. Later, when the truth about Rebecca comes out, the way in which she dies threatens Maxim and Mrs. de Winter's marriage. Whether or not you are a murder victim, if you threaten our leading lady, you are officially our antagonist.
Mrs. Danvers is a nasty piece of work: manipulative, vindictive, and pretty close to murderous. But if you want to see Rebecca's murder avenged, Mrs. Danvers is your gal. We don't know for sure that she's the one who burns down Manderley, but it if it is her, she might be doling out a punishment that the law could never match. Still, she kind of ruins things for our protagonist, so she fits the bill here.