Giovanni's self-centered, naïve, self-deceiving, and unloving personality is most evident in his treatment of Beatrice, who is the daughter of Rappaccini.
Professor Pietro Baglioni warns Giovanni that Doctor Rappaccini's professional character carries "grave objections" in some circles:
It is said of him ... [that] his patients are interesting to him only as subjects for some new experiment. He would sacrifice human life, his own among the rest ... for the sake of adding so much as a grain of mustard-seed to the great heap of his accumulated knowledge.
Yet Giovanni is certain that the doctor's passions for science demonstrate a "spiritual" dedication to his work and should not be a cause for concern. When Giovanni sees Baglioni at a later date, he tries to avoid the man entirely, preemptively dismissing the older man's opinions about the doctor and his daughter. Baglioni notices that Doctor Rappaccini watches Giovanni from across the street, and the way the doctor "fixe[s] his eyes upon Giovanni with ... intentness" is a cause for concern:
For some purpose or other, this man of science is making a study of you. I know that look of his! ... I will stake my life upon it, you are the subject of one of Rappaccini's experiments!
Instead of considering this warning, Giovanni again deceives himself; he certainly doesn't want to lose his opportunity to spend time with the doctor's daughter, so he turns a blind eye to any signs of danger.
Indeed, Giovanni's own senses indicate that something is amiss with Beatrice's nature. He watches flowers wilt in her presence and an insect die from her breath. Naively, he convinces himself that his eyes have simply deceived him in his observations:
He could not quite forget the bouquet that withered in her grasp, and the insect that perished amid the sunny air, by no ostensible agency save the fragrance of her breath. These incidents, however,
dissolving in the pure light of her character, had no longer the efficacy of facts, but were acknowledged as mistaken fantasies.
When Giovanni finds that his own breath has become as deadly as Beatrice's, he doesn't blame her father; instead, he blames Beatrice herself. Lashing out at her, Giovanni calls Beatrice "hateful," "ugly," and "loathsome." Beatrice tries to convince him that her "spirit is God's creature," but Giovanni is heartless in his verbal attack. As she dies, Beatrice accuses him of being far more poisonous than she ever was:
Oh, was there not, from the first, more poison in thy nature than in mine?
Giovanni believes in his own observations and is even willing to dismiss factual evidence to better align with his personal goals. He is naïve to believe that the doctor is harmless, even when he is given ample warning. In the end, Giovanni's heart is proven to be cold and unloving toward Beatrice, who professes that she only wanted to be "loved, not feared."