These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Copy Cat Sue: Shinjiro, especially in the first Shadowchasers fic where he was quite clearly the author's version of Yusei Fuso, right up with giving him a similar deck to Yusei's and a Dark and Troubled Past with Yusei and even having an undefeated streak in the first fanfic, something the author was criticized over. Fortunately this was toned down in Power Primordial wherein he changes his deck and actually loses an important duel.
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 demons and vampires are sickened by your actions, it's clear you've gone too far.
Unfortunate Implications: 7th Librarian changed Tsukihime's name to Tsuki via Retcon after research showed that it is inappropriate in Japanese culture to name a child after a deity.