8. Carol - The Walking Dead
The Walking Dead has always had a problem with characterisation, because arcs are designed to fit the plot, rather than the other way around. It's resulted in Morgan flip-flopping from a nervous wreck who murders everyone to a monk-like pacifist and back again, while even hero Rick Grimes goes from relentless killer to humble farmer depending on what theme of the season is.
Carol's transformation was the one which came out of nowhere the most, though. In Season 1 and 2, she's a meek, vulnerable victim of abuse who needs looking after by the rest of the survivors. Although it's horrible, she's essentially dead weight, someone who can't look after herself, and is totally innocent in a world slowly going to hell.
Season 3 brought her out her shell a little bit but then, suddenly in Season 4, she turned into a complete psychopath. Gone was her good nature, and instead she became someone willing to set sleeping friends on fire so they don't pass on a disease to other characters. By the time Season 5 hits, she's a cut-throat murderer with the killing skills of a Navy SEAL, capable of laying siege to an entire outpost of cannibals single-handedly.
On a whim, the writers transformed her into the most badass gunslinger on the show, a switch so jarring she had to be side-lined from there on out because she'd become too powerful without reason