Other besides the cheating part, i get the feeling his laundering or doing illegal stuff in his company.
It took him 5 years for his business to raise up from the bottom; "not going to the 4th floor" what if thats where he does the transaction since there's no phone signals? (It probably wasn't just a bluff to trapped the guy in the room?).
Girl probably does have something on him and since her dad's wish was for her to marry someone wealthy so she could be taken cared of, she went with him?.. Plus she would've had more 'opinions and input' on the company when she join the guy's company. Probably she didn't loved him at first, but then fell for him?. Maybe she doesn't have anything on him but the guy believes otherwise and is looking for ways to get rid of her like searching for poison?. Afterall, little by little we're seeing what the dad's side-view of what his daughter was like when he was human, but not how his daughter hid her true self from him?. Like, started smoking at 15, drinking when he thought his daughter never drank at all (and god knows whatever else when she was so stressed out, feeling neglected, and depressed from his father's actions and not knowing much about her mother).
All of these are speculations but that guy is definitely fishy!, and as you mentioned, his reactions are way darker than just hiding an affair from his wife. It definitely has to do something with his company business.
Nah guys this man is acting too drastically to have this just be an affair... Something feels INCREDIBLY off and dark about this man for him to be JUST a cheater. I feel like something darker is happening here... Along with the cheating... Am I alone here?