While I enjoyed the story as it was unfolding I liked it even more at the conclusion. Daphne du Maurier does a beautiful job of exploring the complexities of human emotion, perception, and misunderstanding. I feel that this story reinforces the idea that there are no good guys or bad guys. All of us possess a bit of both.
**SPOILERS**
I think Rachel poisoned Ambrose and Philip half-heartedly. Rachel was a creature of survival. She told Philip that when her father died, she and her mother had a very difficult and hard existence. We can piece together that she married Ambrose equally out of necessity and care. She poisoned Ambrose and later Philip because she was corrupted by her confidante and lawyer, Rainaldi. Rainaldi must have thought the effects of poison would impact his lucidity enough that he would sign his estate over to Rachel. Ambrose never wavered, however, and died before Rachel could inherit his wealth.
I believe that Rachel came to truly love Philip while also maintaining a shrewd and calculating desire for security. Philip played into her plans by falling for her and then leaving the estate, family jewels, and money to her. He did, however, make a provision that if she were to remarry everything would revert back to his control. Rachel was delighted with her newfound wealth, of course. Then Philip, spurned on by the happy occasion of righting Ambrose's wrong, asked for her hand in marriage. She accepted and it seemed there would be a happy ending after all. Then the survivalist in Rachel struck out at Philip. She clarified the meaning of the provision about remarriage and suddenly, did not know what Philip was talking about when he announced their engagement to his godfather and Louise. She wanted full control of the money and estate to do with what she pleased, including sending money out of the country to Rainaldi.
Out of nowhwere, Philip was struck down by a terrible illness and incapacitated for weeks. I cannot figure out exactly why Rachel picked this time to poison Philip except that maybe he was causing too much of a spectacle for her and perhaps, thwarting her plans to return to Florence. I also do not know if at any point she meant to kill Ambrose or Philip or if she merely meant to take advantage of their weakened states. Either way, we know the poison killed Ambrose and it might have killed Philip had he not discovered the source of his illness.
I find it interesting that as Philip sends Rachel to her death he cannot find evidence to prove her wrongdoing. In fact, he finds the opposite. There is a letter addressed to her from the bank confirming that they have received all the family jewels to be returned to Philip. And there is a short missive from Rainaldi showing concern that Rachel may have a change of heart and stay in England with Philip.
Initially, Philip feels that Rachel's death is payment for his uncle's but he learns, like the reader, that there is no black and white. In the end, they are both guilty.
Rachel is guilty of killing Ambrose and toying with Philip's emotions. Rachel was strongly influenced by Rainaldi to go to any extreme for their survival but she was also capable of love. She had moments where it was difficult to reconcile her needs and wants. She wavered between reality and fantasy; survival and happiness. Unfortunately, for her, there could be no resolution or balance.
Philip is guilty of sending Rachel to her death and misunderstanding her inner conflict. Once he realizes that she killed Ambrose and attempted to harm him, he is determined to make her pay. This implies some maliciousness on his part but he did not do it with anger. I think he felt that he made a promise to his cousin before he met Rachel and unconsciously, sent her to her death. Still, he regrets his callousness as soon as he is reminded of Rachel's capacity for goodness.