Mecha Sonic from Super Mario Bros Z crashes the Death Egg into Mobius, murders Tails, Amy, Rogue, Omega, Knuckles, Cream, and Cheese simply to fulfill his bloodlust, and turns the planet into a burning wasteland. And this was after he was just created once he merged with several other Metal Sonics. He finds more lines to cross afterwards.
Wyoming was Affably Evil all along, but for some viewers he seems to have crossed the line beyond any sympathy when he killed York. That was in a mini-series, though, so not everyone's familiar with the event.
Tex came very close to crossing the horizon when she proved willing to kidnap Tucker's son Junior and use him to sabotage the aliens' religion, ending the war by genociding the other race. She didn't slip over only because the plan, much like her ship, barely got off the ground, and the goal now seems firmly out of reach. She's also been dead now for longer than she's ever been before, which seems to mitigate the effect somewhat.
Agent South Dakota had already shot one partner in the back, and tried to sacrifice the AI Delta, both to save her own skin. But when Delta then revealed that she had done the same thing to her twin brother, she went from 'bitch' to 'irredeemable bitch'.
And finally (notice these are all Freelancers?), if Agent Washington actually did kill Donut at the end of Recreation, that will almost certainly be his own moral event horizon. No, hedidn't.
Of all people, Princess Zelda crossed this in There Will Be Brawl when she stabbed Link in the back just as he was making a Heel Face Turn, then in the next episode revealed that she had been working with Gannondorf the whole time. Not even attempting to Taking the Bullet for Link redeemed her, really.
Wario. He orchestrated a Pikmin Bomb to take out Red (the only indisputably good character in the series, got his mentally challenged brother to deliver the bomb, and then tried to pay Red's girlfriend off with money (and that was a lie; he had planned to kill her too).
Kirby crosses this in the backstory when he gruesomely murders Daisy.
The SCP Foundation: "SCP-590 is not to be named as anything other then Five Ninety. He is a tool to be used, not someone's friend, sibling, or child. Anyone found forming attachments to an SCP will be removed to a less people-intensive duty." A teenage boy who has the power to heal any human ailment by touching someone and taking the ailment to himself. The way the Foundation took advantage of this is infecting him with mental retardation to keep him pliable. Where they really cross the Moral Event Horizon is that the person who suggested it, Dr. Bright, is his older brother.
There's also SCP-231-7 and the containment process for her, 110-Montauk, which is [REDACTED]. It's so horrible that anyone who performs it gets mind-wiped (if they don't commit suicide first)... and then they do it again...
Cody Jenson was already a dick before, but after his rape and murder of Madelaine Shirohara, followed by killing Amanda Jones while trying to kill Sidney Crosby, the SOTF community in general rallied behind Adam Dodd, whose True Companions the two girls were part of, to kill him.
Adam Reeves manages to cross this line in his very first appearance, after raping Maxie Dasai.
After killing 5 people, Reiko Ishida still managed to cross this line after killing defenseless girl and only living friend, Carol Burke, after Carol said she wasn't as strong as she thought she was.Disproportionate Retribution much?
It also turns out to be an in-universe example; When Reiko comes across her girlfriend Sarah Xu and attempts to get on the escape boat with her, she is denied access to the boat by STAR, who then proceeds to use, among other things, the above incident for a reason why. It results in Sarah and Reiko being separated. Ouch.
Maxwell Lombardi strangling Augustus MacDougal, shooting Harold Fisher, and gloating to the cameras that there was more where that came from. Which there was.
Alex White crosses the line with his slow, torturous killing of Rosa Fiametta.
Aaron Hughes crosses it, but most agree that leaving Tom Guthrie to die at the hands of Nick Reid and then claiming he died to protect the group very early on in the game was quite cruel and confirmed the character to be a Manipulative Bastard in most handlers' eyes.
Marvia Jones in SOTF-TV managed to cross it in her first in-game appearance, when she comes across a traumatized Nate Chauncey. She then proceeds to give her ecstasy (which she tells her is a headache pill) and take advantage of her sexually. It culminates it her calling Nate a whore for accepting her advances, beating her up, and stealing most of her supplies, including her clothes.
In We Are Our Avatars, Carter/Catherine doesn't just cross this after becoming a Team Rocket member, s/he leaps directly over it, starts running, and never looks back. This eventually leads to hir getting kicked off the team and re-defecting to the rest of the group.
Duo mentions an incident with Slur involving seven planets being destroyed, he notes that 5 of those planets may not have been infected with Evil Energy in the first place, and that Slur may have done it just because she could.
Cyber Amy(A.K.A. Amy being fused with her father Deviot's soul) rapes BlackMage.
Deviot revived Kasei. Worked out till Saturn's version of Him decided to give Kasei his power back.
Dr. Horrible's Singalong Blog has an odd example, as the 'hero' of the story passes his before the villain does. Furthermore, he is responsible for the villain's MEH. Captain Hammer's is viewed by many as bragging to Billy that he is going to sleep with Penny solely because he knows Billy has a crush on her. However, if this doesn't qualify, his decision to use the Death Ray on Dr. Horrible in a room full of innocent people definitely does. This results in Penny's death, which tips Billy over the edge.
William Shaw in Paperwalls crossed this when he raped his daughter Cameron, and kept finding new moral event horizons to cross.
Mechakara was already disturbing, and his actions already evil. What really makes him an utter bastard is his new plan on dealing with Linkara: he tries to get him to kill himself by making him think he was the father of the family that killed their own daughter to make a powerful artifact from their dark god. Any leather-pantsing has become utterly impossible.
Alex in Marble Hornets was always a bit unhinged. Breaking Tim's leg, refusing to tell Jay anything, lying to everyone he knows. But the line was crossed in Entry #49 when he murdered an innocent man just checking to see if he's okay, believing it was Jay following him. He then has The Operator dispose of the body. No hope now.
Entry #51 reveals he's been doing it for years, even back when he was making the original Marble Hornets movie.
Parodied in-universe by The Nostalgia Chick's Dark Nella Saga. The Chick only realizes Dark Nella is evil when she founds that Minnesota's big ball of twine has been destroyed. Oh no!
With the Todd storyline, Chick became a concretely psychotic stalker at one of three instances. Either forcing him regularly to be her Captive Date, being too happily willing to rape him, or preying on his money insecurities to make him do a review with her; take your pick.
Throughout To Boldly Flee, the Executor was a sleazy, manipulative, creepy Bad Boss and Magnificent Bastard, but he was hammy. But then he tortured a helpless Luke (one of the youngest members of the site) with his electricity powers and you wanted him to die painfully. Thankfully he did.