When they 'die' and get replaced with some shapeshifter/replica who is secretly evil. JJ Abrams shows are notorious for this. It's nice to keep the actor still involved after their character is whacked but it really ruins everything that character meant on the show to reduce them to some plot point.

I also don't like when an actor leaves a show and they just find some way to kill the character off even if it had nothing to do with the original plans. There's other ways to make the character leave the plot which don't involve having them meet some tragic end.