Though it makes me ask the question - if Rhapthorne can possess someone whilst they're sleeping, is he limited by the amount of sleep they would have? Otherwise he could've just murdered the LHP then been off.
To Neos. Then what's to stop Rhapthorne possessing him the next time he sleeps? Since I imagine he'd have had to do so at least once more, even if he tries staying up.
Plus he seemed capable of forcing people to listen to his speech - and making examples of them and the party. Including his own brother. [He hates you bro]. So given there's no personal connection to the LHP...
I think when he crossed the Moral Event Horizon he was capable of considering murder. He got people out the way for a reason. Certainly at least he ended up culpable as an accomplice in covering it up.
There's that smirk he gives commenting on the LHP dying from 'shock'. Shows some level of benefitting from Rhapthorne's work at least? Which is convenient, cus he can pull a 'POSSESSED' card to assuage himself