YMMV / Perturabo in Remnant

• Crossover Ship: Perturabo x Glynda, Winter and Cinder; yes, really.
• Moral Event Horizon: Salem crosses it when she takes the Chaos Gods up on their offer of power in the hopes of claiming Perturabo for herself.
  • Adam crosses it when he betrays Perturabo and turns the White Fang into a Chaos cult after being "enlightened" by Erebus and Kor Phaeron.
  • Horus himself arguably truly crosses it after the Istvaan Atrocity; about halfway in to the battle, he suddenly realizes thanks to Erebus's slip of the tongue, that he's been duped into being a pawn of Chaos. Rather than try to make amends, though, Horus decides that he's not going to follow anyone ever again and goes about establishing his own empire. The fact that he ordered the massacre of loyalist Astartes, some of which were his own, doesn't even register to him.
  • Doctor Watts doesn't just cross it, he flat-out swan dives over the horizon when he starts engaging in human experimentation to combine Grimm essence with mortal forms. Needless to say, his methods are not pretty to behold.
  • Leman Russ certainly thinks this when he understands Horus lied and unleashed him against Prospero, since Magnus actually didn't betray the Emperor (mistakenly or otherwise) in this timeline, but you may argue it's actually Horus's moment. A more personal and quite jerkass moment for Russ would be the fact he threatened to rape his brother's wife after learning of her existence, just to rile Magnus and make him lose their fight — even he was disgusted with himself.
• The Worf Effect: Tyrian Callows ambushing and killing a squad of Astartes, including a veteran Sergeant. That being said, he had to work for his kills, freely admitting that they are incredibly dangerous opponents and underestimating them is a potentially fatal mistake.