1- A number of our scholars do believe that the Prophet (s) was poisoned, and he died because of the poison. Sunnis also agree the Prophet (s) was poisoned, but they that he was poisoned by a Jewish woman some 3-4 years before he passed away (which we reject, since there is no such poison that kills after 4 years). We believe some of those around him poisoned him after Ghadir.
2- The Ahlulbayt (a) knew they would be killed/poisoned, so why didn’t they protect themselves?
One: they were commanded by Allah not to protect themselves because he wanted them to make a sacrifice. Imam Hussain (a) knew he would be martyred in Karbala, but he still went because he had to make this sacrifice.
Two: a second opinion states that at the very moment when the poison was offered to them, God made them forget this is poisonous and so they consumed it (the fruit that was poisoned).
And there are some other opinions based on various hadiths that we have.