Darvell was actually recruiting Clara into the Brethren from the start.
Because it makes no sense for him to try and recruit Ruthven, who stole his property and left him for dead. He could be a naive idiot, or his recruitment of Ruthven was actually part of a long con. Evidence in his favor:- In terms of their overarching stories, they're actually very similar: naive young people screwed over by Ruthven who turn into vampires to be cured of a lung ailment. Darvell comments on Clara's coughing when he shows up the night after the party, and he knows exactly who she is after so many years, so he knows both her backstory and her health.
- He explicitly does not forgive Ruthven for taking his property, because forgiveness is "a Christian virtue" that he no longer adheres to. Vengeance, however, is likely something he's very keen on; and getting Ruthven's hopes up about being cured, only to have that hope snatched from him, is the perfect revenge for Darvell having been screwed over by him in the past.
- He keeps glancing at Clara when speaking to Ruthven, including a lingering look directly at her immediately after giving Ruthven the map.
- He lets Clara stay in the room when he reveals The Masquerade, which wouldn't make sense if he wanted her to remain ignorant.
- The traits he lists that cement Ruthven's potential for vampirism (particularly the comment about him being a survivor) also apply to Clara.
- He doesn't look at all surprised that it's Clara, not Ruthven, on the beach, and he doesn't get angry at her at all for stealing the map.
- His task for the Brotherhood was to find "a good man" who can be one of the "pointed nails of justice." Ruthven the rapist hardly qualifies, but Clara certainly does.
- If the above is true, then his later long con (pretending to be devoted to the Brotherhood while secretly planning on helping Clara the entire time) isn't a surprise but rather part of an overall pattern.
