Your mileage may vary on this, but in The Fear Hole Professor Melancholly gets this in Episode 7 when he murders the adorable childlike Antagonizer in cold blood. What really sinks this past the horizon is that Antagonizer was telling Melancholly that he loved him.
Hard to say exactly when Cinder crossed this, but for most fans it would be unleashing level 9 Grimm invasion on Vale which came close to destroying the whole city, while commenting on the chaos and how beautiful it was.
Neopolitan gets her crossing earlier on, in Volume 2. Not content on merely inflicting the Curb-Stomp Battle of a lifetime on Yang, she then goes for a Coup de Gr‚ce on the unconscious brawler with a psychotic Slasher Smile on her face.