Brad

You are correct!!

Brad is the killer.

Brad had the knowledge that Jill was deathly allergic to pineapple and chose to poison her with it. He knew Jill was too drunk to realize he bought her a pina colada dollarita and would drink it during a toast. One gulp and Jill was dead within minutes. His ultimate motivation for killing her was that he felt controlled by her. She would not allow him to have a Tinder and wanted to end the relationship, which gave her more control in the relationship. Brad is not a person that likes to feel out of control in his life. The events and fight of two days ago proved to Brad that Jill did not care for his career and his important meeting. He was reprimanded by the boss in front of colleges for his tardiness and it  caused him a great deal of embarrassment. Brad felt invincible because he is a well connected soon-to-be lawyer who could play off the murder an accident: giving Jill the wrong drink. However, there was no accident in his actions. He intended to kill Jill.