It could be anyone

"To anyone on their flight out of New York, they appear to be five best friends excited for a destination wedding in Miami. No one would guess that each of them has a reason to want the groom dead. Trevor Vaughn, the groom in question, wooed his bride-to-be by first becoming close with her friends--which is to say that he learned all of the five's darkest, most dangerous secrets and blackmailed them into convincing Fiona to say "I do." The friend...