Manfred von Karma killed Edgeworth's father because he tarnished his perfect record, adopted Edgeworth, trained him to be a prosecutor, then set him up to be convicted of murder. He was the prosecutor who was trying to prove him guilty.
While we all knew Dahlia was a Bitch in Sheep's Clothing from the moment she popped up, her status as a monster isn't fully confirmed until the flashback case where she convinces Terry Fawles (implied to be mentally disabled) to kill himself because he doubted that she loved him. This is after framing him for murdering her and later her step-sister, and this is just as he's about to give testimony that might exonerate himself and convict her.
Matt Engarde crosses this long before he actually commits a murder, when he, after essentially throwing away Celeste, who loved him, causes her new fiancé, his rival Juan Corrida, to break up with her by revealing that she was once his girlfriend, leading her to be Driven to Suicide. Juan's role in said event could be considered his own Moral Event Horizon crossing as well.
The Walking Spoiler villain of Dual Destinies, the phantom, has done so many things that we don't know of that it's hard to say when exactly they crossed it. But during the events of the game, it's either killing a police officer and impersonating him for over a year so he could corrupt the justice system, sabotaging two rockets and killing two people, then trying to pin it on the daughter of one of the victims, or bombing a courtroom and trying to kill everyone in it just to get rid of one quite far-fetched piece of evidence. And they don't regret any of it.
Quercus Alba ordering the death of Cece Yew (which doubled as Manny Coachen's as well since he was the hitman and what allowed him to become an Asshole Victim) in Ace Attorney Investigations: Miles Edgeworth and Blaise Debeste, Patricia Roland, and Di-Jun Huang's body double having the real Di-Jun Huang assassinated on a secret trip to visit his orphaned son in the sequel.
And then in the most recent game Spirit of Justice, we have the Big Bad, who makes almost every other example here under Ace Attorney look like kiddie film villains. Ga'ran not only kills Apollo's biological father, but she is perfectly okay with letting baby Apollo burn alive in the fire. During that fire, she smears the noble Dhurke Sahdmadhi's name and makes the entire public think he killed Amara and causes them to have a severe hatred of defense attorneys. Not only that, but she passes a law that causes every defense attorney who tries to defend a client in the kingdom of Khura'in to get the same sentence that the suspect would have meaning she's sent hundreds of thousands of innocents to the gallows. And then she even murders her own husband and threatens to kill Amara (who it turns out wasn't dead) and Rayfa should Nahyuta not comply with what she's done.
Monokuma takes a flying leap over the horizon and keeps right on going when he kidnaps 15 innocent students, forces them to murder each other in order to escape, and then brutally executes a student when she dares to defy him. The Reveal that said student was the mastermind's sister only makes it worse.
In the spinoff game, Absolute Despair Girls, it would be far easier to count the times where Monaca isn't crossing it. To pinpoint the spot where she first crosses it, her complete disregard of Masaru's apparent death.
Korekiyo crossed the MEH long before the killing game even started, murdering close to one hundred girls so his dead sister can have friends in the afterlife. And even if that wasn't enough, there is what he did in his Love Hotel scene.
In-universe (and some people out of it) consider Kokichi to have crossed this when he manipulates Gonta into murdering Miu, particularly as the former has no memory of this happening when it's revealed. However, later on he pulls a Villain's Dying Grace and Death Equals Redemption by allowing Kaito to kill him in order to create an unsolvable murder, thus ending the killing game and also saving Maki from being executed.
The true masterminds of the game basically crossed the MEH by having 53 previous killing games. If we assume that each killing games has about 15 or 16 characters, then about 800 lives were lost all for the sake of good television.
Keisuke crosses it in the end of the Special Bad Ending where he uses all the girls, except Kanae, who he killed, as depraved Sex Slaves: Rika is forced to act like a dog, Byakuya his "work of art" that he tattooed and he constantly whips, Natsuki is his "lewd cow" whose mind have been fried by a powerful drug, and her pleasure nerves were extended to such a point that she is in a constant state of delirious pleasure, and even a slap makes her feel good, and Nemu serves as a trophy in the wall that Takato occasionally uses. The first three are also pregnant, and is heavily implied that Takato is impregnating them constantly... yes, it's thatkind of bad ending.
Screw you, Erika!. The exact moment for most people differs, but if you haven't felt she crossed it before, there's a line she says in the 6th Episode that hurls her across this line:
Erika: "I dashed all over the mansion, visited all the 'crime' scenes, and...completely severed their heads... All five people I killed......were very much alive and faithfully playing dead until the moment I killed them."