It turned out to be a trap. Heathcliff knew Cathy had a good heart and took advantage of it. As soon as he got Cathy and Ellen to Wuthering Heights, Linton calmed down. Cathy still loved him, though.
Heathcliff wouldn’t let them go because he wanted the two to be married by morning in case Linton died too soon. The only thing Cathy worried about was the thought of her father sick and having to worry about her on top of it.
Both Ellen and Cathy were imprisoned in an upstairs room that night. Cathy was let out in the morning, while Hareton tended for Ellen for the next five days.