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.
Adam crosses this when he begins torturing Blake to provoke Yang, and then slices Yang's right arm off. Worse, when Blake gets between him and the now unconscious Yang, he tries to decapitate her.
Hard to say exactly when Cinder crossed this, but for most fans it would be shooting Pyrrha through the heart and then incinerating her body until only her tiara is left.
While the jury's still out on Emerald, Mercury can be argued to have crossed this as it can be inferred that he allowed Ruby to get out into the stands just in time to watch one of her friends be dismembered by another.