Who is the antagonist in "Rappaccini's Daughter"?

In order to figure out who or what is the antagonist, we must first decide who the protagonist is and the nature of that character's main conflict. I would argue that Giovanni Guasconti is the protagonist, even though the title describes Beatrice Rappaccini, because he is the character whose experiences we follow throughout the story, the character who undergoes some change as a result of the events. In the text, Giovanni experiences conflict with Rappaccini, with Beatrice, with Baglioni, and with himself, but which is the most significant?

Near the end, Giovanni seems to believe that his conflict is with Beatrice, but she truly loves him and has played no intentional part in her father's plan to change his nature. Finally, Giovanni realizes her innocence, and how much she loves him, and so he offers her Baglioni's antidote, for them to "quaff ... together, and thus be purified from evil." However, Beatrice insists that she drink first and he "await the result"; she still seeks to protect him. As she dies, she asks Giovanni, "Oh, was there not, from the first, more poison in thy nature than in mine?" Giovanni could not see Beatrice's goodness, just as he cannot see his own poisonous nature; he would try to change and manipulate her, just as her own, horrible father has done. Therefore, I argue that Giovanni himself is the antagonist of the story because it is he who eventually brings about his own moral ruin as well as her death.

Who is responsible for the death of Rappaccini's daughter?

Over the course of "Rappaccini's Daughter," both the protagonist Giovanni and the audience learn that Rappaccini's lovely daughter Beatrice has been brought up on poisons. Rappaccini did this so she could tend the poisonous plants in the garden without being harmed. Doing so helps him further his scientific research, but it also makes Beatrice's presence toxic to others. She is essentially confined to the garden and unable to live a full life among other people without harming them.

This state of affairs enrages Giovanni, who has come to fall in love with Beatrice. He goes to a doctor, who gives him an antidote to give to Beatrice. Giovanni hopes that the antidote will cure Beatrice and allow her to leave the garden with him so they can live together. However, when Beatrice takes the antidote, she dies immediately. The poisons that would kill other people have become part of her, and to purge her of that also purges her of life itself.

Naturally, the ending of the story is ironic. Giovanni wants to save Beatrice, but ends up killing her instead. Much like Nathaniel Hawthorne's other short story "The Birth-Mark," by trying to rid his beloved of an imperfection, the lover only kills her.