Peter said to Clarissa that he was in love with a married woman in India named Daisy. He says he is in love but after leaving Clarissa's house he sees a lady walking along the street and just decides to follow her?! He follows her while watching in a lecherous way. He follows her all the way to her apartment while 'observing' her. If he was really in love, would he stalk innocent women and find where they live? I am not going to get into the later stuff when he actually realizes he might not be in love but just this incident alone says a lot about Peter.
He always carries a pocket knife because of his insecurities and is constantly flipping it open and closed and playing with it. He may be in London right now, but I am pretty sure that he did this in Bourton as well. The book is constantly in these nice peaceful settings and for the most part everything fits in with that setting. Except for Peter. With his pocket knife and his stalking (I'm assuming this is not the first time) habits, Peter does not fit with the rest of the book. I suppose you could say he is a 'free spirit' but I think this is crossing a line. What will he do next? I think this is why Clarissa really rejected him.