Beatrice is mortified when Giovanni accuses her of having deliberately entrapped him and turned him into a poisonous being like herself. When he exhales into a cloud of little bugs and twenty of them fall, dead, to the ground, she realizes what her father has done. Rappaccini has wrought his "mortal science" on Giovanni in order to give his daughter a companion. Upon this realization, Beatrice cries to Giovanni, "'Yes; spurn me! -- tread upon me! -- kill me! Oh, what is death, after such words as thine? But it was not I! Not for a world of bliss would I have done it!'" She is eager to prove her innocence, to impress upon Giovanni, the man she loves, that she would never have sought to manipulate him or corrupt his body in this way. He has spoken to her so harshly that death seems like a blessing to her.
Further, when Giovanni presents her with the vial of potion mixed by Baglioni, the potion which he's been promised will rid the poison from his system, she happily drinks it on one condition: she says, "'I will drink -- but do thou await the result.'" She seems to intuitively know that this potion will have a deleterious effect, and so she drinks it, perhaps knowing it will kill her, to save Giovanni's life and to prove that she would rather die than to harm him.