If you've ever been a bullying victim, Iona can be this.
The Pretty Cure Graveyard is basically a spine-chilling display of every single defeated Pretty Cure. Phantom the Pretty Cure Hunter did all of this as a show of his sheer power...multiple times.
Episode 32 has a good part of the battle with Namakelder completely without music, with only the sounds being audible. The colors are also very faded, almost grey. All this give a sense of uneasiness.
Red trying to corrupt Cure Lovely can be this too.
If you ever see Lovely doing this, you had basically stumbled upon a killing machine in a form of a magical girl.