After Baglioni warns Giovanni about Rappaccini’s immorality and his professional stature as a scientist, the narrator uses dramatic irony to foreshadow Baglioni's cruel machinations at the end of the story:
[Giovanni] might have taken Baglioni's opinions with many grains of allowance had he known that there was a professional warfare of long continuance between him and Dr. Rappaccini, in which the latter was generally thought to have gained the advantage. If the reader be inclined to judge for himself, we refer him to certain black-letter tracts on both sides.
Here, the narrator gives the reader a key piece of information that Giovanni lacks—namely, that Baglioni’s condemnation of Rappaccini is rooted in jealousy. As a result, his assessment of Rappaccini is likely to be biased. In the hopes of bringing down his professional rival, Baglioni has a motive to harm Beatrice, and Giovanni is completely unaware of this dynamic. As a result, the dramatic irony here leads the reader to suspect that Baglioni is plotting something—and that Giovanni will fail to anticipate it. Moreover, the narrator’s observation that Giovanni “might have” acted differently had he been given this information reveals to the reader that he will make a fatal error due to his ignorance. This heightens the ominous mood of the story and foreshadows its tragic ending.
