Let's look at what we know about the circumstances of his death also:
Quote:
he was suddenly met with the eerie, unsettling smile of a girl. He instinctively realized the person before him was not his ally and drew his blade, but it was already too late; the girl started chanting and the next thing the man knew he was floating in the air unable to move, and before he could react any further he felt himself being ripped apart. But even that feeling left him only moments after, and he would never move ever again.
The murderer in this instance is a female character. If we're telling the truth about who we are, then Fynn (Noctis), Formy (Kefka) and myself (Cid) can't be the killer. Although we could still be the accomplice of course.