I like this episode (except for the button-pushing-happy Columbo), but never really understood Kay's motive. It's never really made clear. This episode and The Most Crucial Game both fall victim to this.
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?