There are few tropes that I hate more than, “I don’t give a damn, I’ll do what I want,” Character Derailment, where in a character changes drastically for reasons that have nothing to do with character growth and have zero basis on previous characterizations. There are three major reasons why this happens:
- Characterization to Fit the Plot: In soap opera style situations, where the focus in on shocking the audience at every turn you always have a character whose characterization changes to fit a given plot point. They often start as villains, but can easily turn into heroes and back again, simply because somebody has to play that role at any given moment. This turns the character into a silhouette to be filled by the writer(s) as they see fit.
- New Writer on Board: In long lived IP, like soap operas, long-standing movie franchises, and comic books, suffer constant staff turnover. These new writers may not understand the existing character(s) or have tastes that vary from their predecessors. They might like or dislike a character and change their characterization accordingly.
- The Fans Lens: Common in fan fiction, fan writers, like their professional counterparts above (and many of those were fans first) tend to zero in on one particular phrase, thought or action by a character at the expense of the rest of its characterization or like the first point above, they create works where in characters act wildly out of place to fit their own plot ideas.
Mind you, a good explanation coupled with changes based on the internal logic of the fiction can (and often do) help avoid this odious trope.