Nick

You are incorrect!

Nick is not the killer.

While he disliked Jill a great deal, killing her would mean she would be the center of attention (yet, again). This was Nick's big night and all he wanted was for Jill to stop making it about her. However, she had to go and get murdered.

Mr. Conway, The Bartender

You are incorrect!

The Bartender, Mr. Conway, is not the killer

The unknown individual he was texting and who he met on his smoke break was his bookie. Mr. Conway is deeply in debt with this person and is afraid of what will happen if he doesn't pay up. He fears that if he is seen talking to the police his bookie will think he is turning him in. Sure, Mr. Conway did not like Jill and really couldn't afford to lose his job because of her, but his gambling debt and the problems with the bookie are all that's on his mind.

Megan

You are incorrect!

Megan is not the killer.

Despite their differences, Megan loved Jill, and they were still best friends. Megan believed Brad would eventually leave Jill for her. She was willing to wait. She was frantic and occupied by her phone because of the accident and massive amounts of patients at the hospital. Colleagues kept calling and texting her to come back into work. She was trying to hide this from Nick because she wants to be there for him and not ruin his celebration.

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.