Isabella is the most mysterious character of the season. As a young woman who has moved around her whole life and has diplomats as parents and quite a juicy (most likely illegal) history, she is a great candidate to be the murderer.
In July 1999, Isabella arrives at the Landry household, hoping to start over in a small town. She immediately takes an interest in Luke and asks Megan if she can make a move on him. Megan initially lies about her feelings for him, and Isabella and Luke kiss and hang out like a couple. It is then revealed that Megan and Luke are dating by December 1999. This could be her motive. She likes Luke and makes her move, and as soon as they start getting close, Megan reveals her feelings, and Isabella backs off. However, those burning feelings of jealousy don’t go away so quickly.
We know Isabella has a dark past because of her suspicious stories, that she attended three different schools in three years, and other unknown secrets. In July 1999 in Episode 5, Trevor (Olly Sholotan) surprises Isabella and mentions the murder of his sister and her best friend, Lisa, to whom she had been writing letters (and not sending them). She drowned in St. Barts, and though nothing is explicitly said, it is incredibly telling of what she could be capable of. (As Trevor puts it, “You know Iz. She’s capable of absolutely anything.”) Isabella’s lawyer also confirms that she had to hide something like this before, referring to Lisa’s drowning, and it was covered up because of her diplomatic immunity.
In July 2000, when Luke’s body is found, she immediately turns to Isabella and says they need to get their stories straight, suggesting they were involved somehow. Her backpack is discovered at Luke’s cabin with a lot of money inside and her prints on the gun, though she claims to have not been there since July 1999.
In Episode 6, Megan finds the police reports that say Isabella convinced her friend Lisa to go swimming while drunk the night she died, along with the fact that she fled St. Barts overnight by helicopter after that incident.
In December 1999, Luke tries to kiss Isabella at The Plunge, even though he’s with Megan, adding to her motive to kill Luke. The question is, would she kill Luke so neither of them could have him or so she could have Megan all to herself?