When the Thorpes can't convince Catherine to cancel plans with the Tilneys for a scheme of theirs instead, John Thorpe goes and lies to the Tilneys and does it for her. When she protests, they actually physically restrain her for a few minutes. As soon as she's released, Catherine promptly turns around and runs after the Tilneys to set the record straight, refusing to let tricks or lies cheat her out of what she really wants. If it was just for what she wanted, she might have done it, but she did it because of her morals. She refuses to break her morals.
Catherine instantly seeing through Isabella's letter claiming she's still in love with James, and declaring their friendship over.
Capt. Frederick Tilney, older brother of the man with whom Catherine falls in love, easily seducing Isabelle away from Catherine's brother and what would have ultimately been an unhappy marriage, and then promptly breaking the engagement with her. His motives are never specifically mentioned, but, whatever his purpose, he has just saved Catherine's beloved brother James from a gold digger without becoming ensnared himself.