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.