Ah, Winterbourne. The biggest hypocrite ever.
Cases in point:
1. He tells Daisy it's inappropriate to be seen alone with gentlemen, and yet he takes her on a whole day trip alone with him to a spooky castle.
2. He makes fun of Italians like Eugenio and Giovanelli for having elaborate mustaches, when he has a mustache that he's fond of twirling at times, so we figure it must be at least a little elaborate, too.
3. He tells Daisy not to be such a flirt then says he wishes she would flirt with him.
4. He tells Daisy not to have secret romantic relationships when he has a secret romantic relationship.
5. He's an American who totally knocks Americans.
What's the deal, Winterbourne? You better check yourself before you wreck yourself. And he's not the only one. Women in the novel are judged way more harshly than men, which is one of the worst kinds of hypocrisy in our book. There's also a lot of hating on Daisy by the potentially jealous (Mrs. Walker) and increasingly irrelevant (Mrs. Costello). Hypocrisy just looks bad on everyone.
Winterbourne's self-contradictory behavior and beliefs are not the result of deliberate hypocrisy, but rather of his failure to know himself.
Daisy's downfall is her failure to perform the divide between public and private behavior that American necessarily adopt when abroad.