The Autistic World of the Autist: It's not entirely known when Imaginee crossed the line but one notable candidate is when he blows up Superautist's homeworld and then given Batautist a Fate Worse Than Death. He may have crossed it as a child when he beaten Roobert's pet puppy, Buddy, to death. Thankfully, it turns out Buddy is Not Quite Dead, although it's clear Imaginee did not intend for Buddy to survive.
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.
The most despicable character in Gotham Girls, a web-exclusive tie-in to the DC Animated Universe, is surprisingly not any of the supervillains but rather Acting Commissioner Greenway. Throughout the series she commits numerous violations of police ethics, notably trying to kill criminals from the get-go instead of attempting to arrest them first, but the point at which we realize how twisted she is is when, after Commissioner Gordon returns and she has to step down, she takes out her rage by ordering Catwoman, Harley, and Ivy beaten to death in prison. Thankfully, they escape.
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 the reason for that last incident? He got an AI, and she didn't.
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.
With Season 10 rolling out, Sigma, Maine's AI seems to have gone down this path. He, along with Gamma, coerce Carolina to taking two AI's so she could beat Texas, he manipulates Maine into becoming the Meta, and is, in essence, responsible for Project Freelancer's infighting and eventual implosion.
The Director of Project Freelancer, with 'support' from Omega & Gamma, can be said to have crossed it when it's revealed they horrifically tortured the A.I. 'Alpha' - known today as Church (!) - to the point where the thoroughly-broken Alpha actually started shearing parts of himself off in a vain attempt at self-preservation, all so they could harvest those parts and make them into AIs for the Freelancer agents. It's even acknowledged as an MEH-crossing in-universe, as the Chairman later says that they'll be rewriting entire sections of the moral & ethical codes for dealing with AIs as a result of what the Director did.
Come Season 12, and Felix, of all people, gets one. After ordering Tucker to throw him a grenade meant for Locus, Felix tells the Reds and Blues that he and Locus are still working together and were hired, not by their respective factions in the planetary civil war, but by an unknown third party. Not only that, they're not trying to end the war, they're trying to prolong it so they can kill every single person on the planet.
The Season 12 finale ultimately gives this to "Control", the mysterious employer of Locus and Felix. While their plans were already morally reprehensible, The Reveal that his identity is Malcolm Hargrove, aka the Chairman, makes this worse when you take his prior actions into account.
Adam turns out to have a lot of issues with Blake for abandoning him during the Black Trailer. He tells Blake he intends to destroy everything she ever loved for having the temerity to walk away from him. He stabs her to provoke Yang, then slices Yang's arm off. When Blake tries to shield Yang's body with her own, Adam victim-blames by asking her why she has to hurt him, then decapitates her. It turns out to be one of Blake's shadow clones to give her time to flee with Yang, but the illusion fooled Adam and he didn't hesitate to decapitate her.
Jacques is by no means a sympathetic character, especially concerning his parenting and how he seemed to drive the Schnee family name to the ground, but what can really convince us he's bad news is how he slaps Weiss, keeps her prisoner,note it's implied that he'll never let her out, so it's more severe than just grounding her and proceeds to disinherit her simply because she stood up for Vale and Beacon when the denizens of Atlas mocked it. Considering that Weiss called out the people of Atlas for not caring about a battle that claimed dozens if not hundreds of lives and set the Grimm on the loose, and Jacques still takes their side over that of his own daughter, this is the action that proves beyond a doubt that he only cares about himself.
Mecha Sonic (renamed Metallix in the reboot) 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.