Not sure how much darker he could get. Wasn't there one where Hunter killed all the girls and fucked their corpses? I recall an image of him laying on a bed with all the girls dispersed throughout the room covered in cum. Or were they just drugged/roofied? And this episode we had him literally kill three people in cold blood like it was nothing.
Not to mention that that specific bad end implies Hunter has killed before- enough times at least that he has a "guy" to handle clean up.
Oh I had a possibility for an ultimate
Ultimate NTR bad ends in the final chapter (not the next one but the one after it) much darker than we have already seen.
I would say that Hunter sucesfully frames Sterling and has him exiled out of the house. Sterling now tries to make sure that hunter does not corrupt the girl and take away their future. He remains in Contact with Lucia that assure him that she will counter Hunter's tactics from inside of the house. Sterling manages to keep Hunter away from his loved ones with her help.
Months later, Ophelia will offer an olive branch and invite him inside the house again where she will agree to listen to what Sterling has to say about Hunter. Sterling will enter the house and the girl will be polite and nice when they receive him.
Sterling will present new proof that Hunter was really the one drugging everyone and they will all be shocked by such revelation. They will all apologise for how they treated him and exiled him from the house when he was only trying to protect them. They will offer him to supper with them since now he can safely return to the house. Sterling will be relieved that he has his family back, he will eat, drink and then will feel sleepy in the middle of the supper and will go on the couch.
When Sterling wakes up, he realise he is being chained, unable to move and the girl all look at him with ominous eyes. Sterling is unerved and ask what is happening... Then Hunter enters the room and ask the girl to greet him with kisses and grope his crotch to which they immediatly obey.
It will be revealed that Lucia as long been corrupted by Hunter soon after Sterling left and she was lying when she said that Hunter was kept under control. All of those tactics were only an illusion to keep Sterling quiet.
And now the girls have been corrupted to an inimaginable degree. Now they are as bad as Hunter, they are willing to twist the knife into Sterling hearts by their own will.
They are looking at him in the eyes and tell him straight up that they no longer care about him, that hunter is NOW their master and that Sterling will never be intimate with them ever again even after Hunter's death. Of course they use subtlety in their words (would be preferable) but the intention from their communication needs to be clear.
And for the rest of his life, Sterling will have to watch his women be fucked and impregnated by hunter with the full understanding that it's not simply their body who are addicted to pleasure, their mind as been twisted in order to make Sterling suffer by Teasing him endlessly but NEVER giving him the satisfaction of orgasm. They will make sure he will remain prisonner for the reminder of his life even after Hunter's death and will always deny him any sexual satisfaction...
THAT would be the ultimate corruption and I would say, the ultimate bad ends.