9. Agent L - Men In Black
At the end of the first Men In Black film, Will Smith's agent J is forced to Neuralyse his mentor, K, before hiring medical examiner Laurel Weaver to be his replacement. She apparently gets the job thanks to killing Edgar, despite her having precisely none of the attributes it took J to get his black suit.
Why She Was Written Out
There was initially a rumour that Linda Fiorentino wasn't in the sequel because Tommy Lee Jones refused to work with her again and another suggesting she priced herself out of a return. She says herself that she was simply unavailable because of a scheduling conflict, but given that her only film around the time was Liberty Stands Still, you'd think she would have made space for her one true breakout hit?
How She Was Written Out
Regardless of the why, the how was a disgrace. After having her eyes opened to the wonders of the galaxy, L apparently simply got bored of it and yearned for a simpler life once more, according to the MIB Wiki:
“Now, she is probably once again the same sexy and cynical morgue examiner that she once was with no idea about the existence of aliens, although she probably still gets neuralysed on a regular basis, as she probably continues dealing with alien corpses.”
Imagine a world where someone sees that world and wants very quickly - not through fatigue or over-saturation as with K - to simply not know about it anymore... It's baffling.