What did Rappaccini want to do? Why had he grown poisonous plants in his garden?
Doctor Rappaccini is said by Professor Guasconti to have a great devotion to science, a greater devotion to science than to people, and he is willing to sacrifice people, including himself, for what he considers the greater good of the advance of knowledge. All of his plants are a kind of experiment, as he looks for cures for diseases and adds to the body of scientific information. He has a theory that plants contain within themselves not just poisons, but also potential antidotes for illnesses. And in fact, Doctor Rappaccini's daughter, the beautiful Beatrice, is herself one of his scientific experiments. She has been raised to be nurtured by the poisonous plants he grows, and Rappaccini has created a daughter who is now impervious to their poisons. It is implicit that Rappaccini has meant to confer on her some form of immortality, since the plants seem to protect her from illness. At the end of the story, Beatrice rejects this, dying rather than living with the poisonous evil her father has wrought.
It is interesting to note that Rappaccini was not exactly wrong, since we are learning or perhaps rediscovering that there are plants that can hurt us or cure us. But one moral of the story is that one can tamper too much with Mother Nature, and this tampering results in evil and death. Another is that when one elevates science over mankind, this is another form of evil, and in fact, there have been many unethical scientific experiments done because of this. The Tuskegee Project is an excellent example, allowing men to die after a cure is discovered, all in the interests of "scientific knowledge."