Amory Wars'Big Bad, Wilhelm Ryan, and Dragon, Mayo Deftinwolf, crossed this right off the bat. Ryan orders the titular characters' children killed because one might grow up to be The Chosen One. Mayo gleefully manipulates Coheed into doing the deed, making him believe his children are going to go Axe Crazy and destroy the universe because of a (nonexistent) virus. And all of this transpires in the second song on the first album. One survives, and grows up to be a rather dark and cynical Messianic Archetype.
In The Rake's Song by The Decemberists, the titular character sings of how he married young and left his licentious life behind, only to come to regret it round about the time his wife had their first child. When she dies in childbirth and he gets stuck with the remaining kids, he decides that getting his old life back is a simple matter of Offing the Offspring. He happily explains how he did his three children in, and then gloats that the listener would expect him to be haunted with guilt from these acts, but really he's not that bothered. In the proceeding songs, he kidnaps and rapes Margaret, with the help of the Forest Queen. He gets his comeuppance in the end though.
Then the Forest Queen herself crosses over when she flies The Rake across an uncrossable river after said abduction of Margaret, so that her adopted son would never leave her for Margaret.
Especially when she's sworn an oath to her son that he can have one night with his love before he has to come back to her forever anyway.
Another Decemberists example is "The Culling of The Fold", in which the narrator encourages others to engage in their own Moral Event Horizons such as torturing and murdering their closest friends and lovers. It's possibly a companion to the song "Shankhill Butchers" which Meloy specifically said that was about the story of the Shankhill Butchers. Their story was told to children to frighten them into behaving.
Edward Baynes in Dream Theater's Metropolis, Pt. 2: Scenes from a Memory crossed this line when he shot and killed his brother Julian and said brother's lover, Victoria, and got away with it. And just when you thought it was over, the Hypnotherapist, implied to be his reincarnation, kills Nicholas, Victoria's reincarnation. Like he couldn't resist killing someone once...
Eminem's song "3 AM" deals with how a serial killer's mind works (a la the characters in slasher films, which he portrays in the song). In the final verse, he explains when he crosses the threshold by explaining when his "days of serial murder manslaughter begun". He describes when he dismembered his cousin, cutting him up and drinking his bath water (when he first thought about drinking the murdered's blood instead).
There are more than a few of these in operas as well. Sometimes the major villain (or Anti-Hero) has to do something really bad, otherwise people might still sympathise with him.
At the start of Wolfgang Amadeus Mozart's "Don Giovanni", the title character - already portrayed as somewhere between a Magnificent Bastard and Affably Evil - first tries (and fails) to rape Donna Anna offstage, then murders her father onstage while trying to escape after she's fought him off. Later on, he shows his Affably Evil side by trying to seduce the village girl Zerlina - on the day before her planned wedding, no less. This actually fails when his ex-lover (or ex-wife, if you believe her claim) shows up. He throws a party at his home, invites the whole village as a distraction, drags the girl offstage and has his wicked way with her. And that's just Act 1... in Act 2, whilst still on the run, he manages to humiliate the ex-lover by setting her up with his servant disguised as himself beat up Zerlina's husband-to-be: and, off-stage, seduce the wife of his own loyal servant Leporello. And brag to Leporello onstage about it afterwards. And then, mockingly, invite the statue of the man he murdered, to come to dinner. Unfortunately, that didn't end well for him... but the statue gives him a chance to repent of his evil ways, and Don Giovanni *refuses*. He knows perfectly well he's crossed the Moral Event Horizon - he's talking to the statue of the man he killed when he did it.
In Bizet's "Carmen", Don Jose actually LOST the real reason for his moment of Moral Event Horizon Crossing in the opera, as compared to the original novel that the opera was based on. In the book, he actually kills his commanding officer, in the fight over the eponymous Carmen: at the corresponding point in the opera, the fight is broken up by Carmen's smuggler friends (who arrived too late in the book), and Officer Zuniga is still alive when dragged offstage by them. (His fate would seem uncertain, except he has one line in a crowd scene an hour later to reassure us he is still alive - although the line could equally well be sung by his deputy, Sergeant Morales.) Nevertheless, Jose decides he has no choice but to join the bandits (rather than remain behind and face the music from his still-alive officer.) Part of Act 3 of the opera revolves around references to his aging, sick mother (off-stage), the only person left who believes he might be redeemable... He ends the opera by murdering Carmen and turning himself in to the guards. But it's kind of less clear when he crossed the MEH although he certainly did at some point.
In Beethoven's "Fidelio", the corrupt governor Pizzaro is already pretty evil - imprisoning political enemies after show trials, and in one case no trial at all. But his Moral Event Horizon could be ordering the murder of Florestan, in the prison, to prevent Florestan (the one who never got a trial, and had evidence of Pizzaro's corruption) being found or freed by the new Minister of Justice. His jailer, Rocco, is offered the chance to cross the Moral Event Horizon himself (by doing the murder), for money, but refuses - it is not clear whether out of cowardice or principle - and agrees only to dig the grave and let his boss do his own dirty work. Rocco even later does a Heel-Face Turn, at a point where he technically could have got away with backing his boss (by picking up the pistol dropped by "Fidelio" and pointing it at Pizzaro instead of handing it to him, just before the Minister arrives for the prison inspection. To his credit he doesn't even seem to realise that there was an alternative option.)
Wagner's "Ring" cycle has one Moral Event Horizon after another. Alberich crosses it at the start by stealing the Rhinemaidens' gold: Wotan and the gods cross it by stealing it (and the eponymous ring) off him purely because they need to pay off a debt: Fafner, the giant they pay off, kills his brother to keep all the treasure. Siegmund, the one who should have been the gods' Designated Hero, crosses it by committing incest with his sister, and thus must die against Hunding rather than being allowed to defeat him: Brunnhilde crosses it herself by disobeying Wotan's orders and protecting Siegmund, and as a punishment is stripped of her status as a Valkyrie and forced to become mortal. Even Siegfried is tricked into it by betraying Brunnhilde and not only marrying Gutrune but forcing Brunnhilde to marry Gunther. It's repeatedly made clear that there's no way back for any of them, although Brunnhilde is the only one to actually realise the situation, seize the moment and turn it into a case of Redemption Equals Death, by throwing the Ring back into the Rhine, to its only rightful owners the Rhinemaidens (thus breaking its curse), and immolating herself on Siegfried's funeral pyre.