Don't worry about the motive,what about how quickly she put together the murder plan,she goes from loving Mark one day,then the next she's got this elaborate plan to kill him,was this always a fallback plan if she didn't get the job?
Jealousy Jilted Job that she wanted By killing him she punished him for accepting the job without negotiating an offer or partnership for her, and also she got the job she always wanted.