Moral Event Horizon/Theatre

The titular character in Macbeth (the play for which the page image is an illustration) reaches this point when he has Macduff's family, including the kids, murdered.
Medea. She convinces two kids to cut up their father and put the pieces in boiling water, making them think it'll make him younger. And that's even before the scene when she puts the children she had by Jason to the sword.
- She crossed it even before they got back to Argos when she chopped up her younger brother and tossed the pieces into the ocean so that her father would have to delay his pursuit to gather the pieces for a proper burial. This was so awful that Jason's intervention was the only thing keeping the rest of the Argonauts from tossing her overboard too.
- Though that only happens in the myth. In the play her Moral Event Horizon is stabbing her two children purely because it will hurt Jason. This is after she kills his wife by lighting her on fire with magic poison. Oh, and then she sails off to Athens under the aegis of the king.
- It should probably be noted, though, that most of these (aside from murdering her brother) weren't included in the story until Euripides introduced the idea that she murdered her children. Up until that point, she was more of a deeply flawed Woobie, Destroyer of Worlds, since Jason was kind of a bastard to her.
In Bat Boy: The Musical, Dr. Parker is a pretty sympathetic character until he murders a kid in order to frame the title character. After that, he's more of a monster than Bat Boy ever was.
Regina in The Little Foxes is greedy and morally bankrupt as it is, but she crosses the event horizon when she lets her husband Horace die of a heart attack because he won't go along with her scheme.
Similarly, Richard III probably hit it when he had the princes drowned.
Caldwell B. Cladwell crosses this when he orders Bobby Strong to be sent to Urinetown, which is actually being thrown off Cladwell Column, knowing full well that his daughter will probably be killed by the rebels. Necessarily Evil or not, that was just unpardonable
Joe Keller of All My Sons has to qualify for this in some measure, though Your Mileage May Vary.
- To put it in perspective: yeah, people might have been shopping faulty parts to the military in World War II for the contract money, but Joe not only did it knowingly, he then pinned the crime on Steve Deever, his best friend and business partner, by pretending he had been sick the day the parts were shipped out when his greed gets 21 pilots killed. Said friend gets life in prison and Joe gets off, retaining the parts business for himself. Not only this but it also causes Steve's own family to turn against him, and Joe has no problem with letting everyone believe he was both a hero who uncovered Steve's incompetence and an innocent victim who had conveniently been sick at home when he wasn't. And he allows this to go on for years. Then when the truth comes out, he's not very remorseful about it and tries to justify his actions and get out of it as being "for the family" and that lots of others were doing it at the time, so if his son Chris was going to turn him to the police he might as well turn over everyone else who did it. Then, when it's revealed that his actions caused Larry, his other son, to kill himself because he couldn't have the shame of what his father did, he goes into the house as if to get his coat so he can be taken into the police to atone for what he did... where he promptly shoots himself in the head just so he wouldn't have to go to jail and be exposed for what he did, or have to deal with the fact his son's death was his fault.
The eponymous man in Phantom of the Opera crosses the Moral Event Horizon when he ties Raoul to a noose and forces a Sadistic Choice on Christine; either confessing love for the Phantom and buying Raoul's freedom, or confessing love for Raoul and watching him die. Lampshaded in Christine's song lyrics: "The tears I might have shed for your dark fate, grow cold and turn to tears of hate!" Though she still pities him by the end anyway, so maybe this doesn't qualify.
Greek tragedy has a term for this event: Harmatia. It's the act the Tragic Hero performs, usually motivated by his "hubris" (Tragic Flaw), that starts the tragedy down the path of no return. Normally this is some kind of offense against the gods or against the natural order: a murder, an act of blasphemy, a rashly-taken oath, etc. But once committed, it can't be undone. The Tragic Hero and every character around him is doomed, no matter how hard they try to escape their fate.
Seneca's play Thyestes is about King Atreus' Moral Event Horizon. Mad at his brother, the title character, for stealing his wife and attempting to steal the throne, he pretends to call Thyestes and sons back from exile and serves Thyestes his own sons for dinner and takes great pleasure in telling him "you ate your children".
Stanley's crossing of this in A Streetcar Named Desire came when he raped Blanche to insanity and then lied that he never once touched her afterwards.
In Wicked, at least as far as Elphaba was concerned, the Wizard crossed the horizon when he broke Professor Dillamond's will, turning him from a respected professor into a mindless animal. For Mrs. Morrible, the horizon mark came when she created the cyclone that brought Dorothy to Oz (and killed Elphaba's sister Nessarosa). Interestingly enough, once Glinda takes the reins of government, she is willing to simply exile the Wizard from Oz, but has Morrible sent to prison.

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