After noting Charlie's response to Walter's ultimatum, what does she come to see was Walter's motivation in making it?

After Kitty sees how quickly and assertively Charlie plans to avoid divorce and a scandal she realizes that Walter guessed that would be Charlie's response all along. She realizes that Walter wanted to expose Charlie's selfish nature. He wanted to Kitty to realize that while Charlie may claim to love her, he would not jeopardize his personal comfort to marry her or even to protect her.