Daggett's most famous victim, Matt Hagen aka Clayface, crosses it in "Growing Pains" when he reabsorbs his own "daughter" Annie, a broken-off piece of himself that had gained sentience and her own set of morals, back into himself then laughing it off. Seriously, it's at that moment Hagen stops being a Tragic Villain and becomes as monstrous as Daggett himself.
"Showdown" is about Arkady Duvall who has already crossed it according to Jonah Hex who has tracked him across twelve states "on account of what he had done to a girl back east" and as if that wasn't enough a barmaid at the beginning of the story tells Hex "He hurt one of my girls real bad." They don't go into detail, but he at the very least violently assaulted them. When the man appears in person, he's whipping one of Ra's workman for slacking, when all the guy was doing was trying to pick up something he'd dropped. When Hex is caught spying on them, Duvall orders the workers to lower him head first into a vat of molten lead, sealing him as a murderer along with abuser.
The Sewer King from "The Underdwellers" crosses the line by kidnapping children to use as slave labor and steal things from him. He trained them to be severely photophobic and afraid of adults. He also scares them with his trained crocodiles and makes it forbidden for the children to make a sound, punishing them by shoving them into a tiny little room with bright lights without food or water for hours — and sometimes, he does it on a whim, even to a kid who managed not to utter a peep despite the extreme scare tactics. In the Animated Series, the Sewer King was one of the few villains Batman had seriously considered killing on the spot, and that's saying quite a lot.
Ferris Boyle from "Heart of Ice" starts crossing this line when he interrupts Victor Fries' cryogenic experiment that can save Nora's life, even though Victor warns it could kill her. But when does he permanently cross it? By kicking Victor into a collection of chemicals that started his transformation into Mr. Freeze and leaving him and his wife to die. Even Batman is horrified at what the newly named Humanitarian of the Year did and leaves him encased in the iceberg instead of breaking him out with the coldly delivered line "Good night... Humanitarian."
