Crothian said:
Only when the player is actually playing it. Then it is called a non player character.
Let me rephrase, it totally depends on the circumstances.
If the GM talks to the player and says "what do you think about the character who you played turning to evil?"
And the player says, "no way! his highest ideal was sacrificing himself for others and protecting the innocent. it would be out of character."
Then the GM should say "you know what, you played this character for X months/years, you're the authority on them, OK. "
If, however the player says "you know what, I never thought of it before, but yeah that sounds neat, go for it, and have a great campaign".
Then the GM should villify the once-good character to their heart's content.
If the GM is running a new group of players and wants to use elements from their old campaign, then there's no reason why they can't decide all the PCs turned into rabbits. They're designing the world after all. Any inspiration the GM wants to use is legitimate.
However, if the GM is running a game which includes the player of the character in question (or possibly could include the player during some...say she returns from a summer vacation), then the GM shouldn't make any drastic changes to the character unless the player has expressed agreement.
I guess I assumed the worst case scenario: Player returns to a long campaign after several months away, and GM says, "Oh, you can't play your old PC because she has turned to the dark side and is the new nemesis of the party. Why don't you roll up a new one?"