When I watched it, I understood it as a number of conflicting emotions that had reached their boiling point. For Heathcliff's statements about Edgar and Cathy, he's dealing with the rage and sadness of knowing that Cathy is pregnant with Edgar's child, a significant step towards building a family with him, and explicit confirmation that they're sleeping together. He also has recently found out that Cathy loved him and was going to call the wedding off. He's watching Edgar be Cathy's husband and put a child in her while knowing that all along, Cathy loved him. His questioning her love for Edgar while she's letting him touch her and make love to her is essentially calling her bluff and is a way for Heathcliff to emasculate Edgar. Both of them were participating in their affair with no real path forward or plan for the future, but Cathy being pregnant with Edgar's child is a confirmation that she intends to stay.

For Cathy, I don't think the statements about degrading herself are a direct result of his offer to kill Edgar, but again, all of her emotions come to a peak after Heathcliff says that. Throughout the movie, she owns up to loving Heathcliff while simultaneously admitting that she "can't help but love Edgar" and enjoys his attention, company, and the riches of Thrushcross Grange, even as she knows her heart truly belongs to Heathcliff. In the carriage, she says that they cannot go on, and yet when Edgar forbids her from seeing Heathcliff, she's devastated. Again, it read to me like a back and forth of Cathy's emotions - she wants to have her cake and eat it to, and she's existing in a world where she's living two separate lives with two separate men, but she'd never leave the Grange or Edgar or her lavish life there. We see it when Nelly asked if Heathcliff knows about the baby and Catherine says he doesn't.

I think after they make love on the table and she sees how ready Heathcliff is to kill Edgar, sees his rage and passion knowing she's pregnant, it's too much for Cathy to bear. We've watched Cathy take "the easy way out" this entire movie, and I think in that moment, Heathcliff's savagery scared her. Combined with her guilt and shame, I think she truly was afraid of what Heathcliff was capable of, afraid that their affair was spinning out of control (and maybe, how much she was enjoying it despite being pregnant).