The Aboleth from the second fic crossed this line a long time ago. It was willing to murder the populations of entire planets for no reason other than greed. It did so at least once, with the homeworld of the Primordial Titans, and tried to do so on at least one other occasion.
In Torment Lorelei crosses it by her rather sadistic act of Cruel Mercy, turning Nichole into a cat after forcing her to try to win an antidote back from her (intending to keep her as a pet once the change becomes irreversible).
Lareth damns himself by turning on his partners, shooting them in cold blood in order to flee and save his own skin. Clearly, what ultimately happens to him is not undeserved.
In Backwater, Solex's in-story crossing is done when he and his entourage beat and gang-rape Kastleneid (his golem servant, purchased for the intent of serving as a Sex Slave) and leaving her to die in a gutter like trash, for no other reason than to blow off steam following his public loss to Tsuki. Clearly, when a demon and a vampire are both sickened by your actions, it's clear you've gone too far.
Uncanny Valley: In ''The Eldritch", the people the Possessor controls move very awkwardly, as though the entity doesn't know how to simulate human movement properly.
The TV series
Complete Monster: Doctor Randall, from "Parts Unknown", is the head of an organ trafficking ring that is operating out of Long Island. Using a health club called the Fitness Factory as a front, Randall fakes the deaths of the club's healthiest patrons by exposing them to a paralytic neurotoxin so that he can later dig up their graves and kill for them real by harvesting their organs for sale on the Black Market. When a pair of teenagers stumble onto the latest grave exhumation, it forces Randall's henchmen to quickly rebury the victim, who suffocates while trying to claw his way out of his coffin; the incident attracts the attention of Jonathan MacKensie and Edgar "Benny" Benedek of the Paranormal Research Unit. Randall is shown personally cutting the organs out of one of his abductees before having her disposed of, and he later tries to gun down both Jonathan and Benny to prevent his criminal enterprise from being exposed by PRU.