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.
Moral Event Horizon: Marguerite plunges head first into this when we find out she kidnaps and kills children to drink there blood. She has no problem killing Sebastian, the Monster, anyone else in the cast or using her lackeys as cannon fodder. Poor Lex.
Isabella crosses this line twice. First when she sleeps with Max, the boy Lex liked at the time, out of spite and rubs it in her face later on. Second is when she tries to kill Lex and Damion.
And Damion crosses it in the early part of the fifth story arc. It starts when he sees Lex's ghost embrace Sebastian and blows up at him for not resurrecting her. Next, he practically keeps Lex under lock and key by trying to keep her at home every night as well as suspecting Sarah of plotting to kill Lex to take her place simply because she came from an alternative universe. (She wasn't.) Things come to a head when he kidnaps Sarah and brings her to Carnival Macabre to have Sebastian send her back and he lashes out at Lex for being angry with his possessiveness, sudden instability and not wanting to continue the relationship in his current mental state. Luckily, Angelique is able to put him to sleep.
They Wasted a Perfectly Good Plot: Besides a brief glimpse into her teen years, we never quite get a thorough glimpse into Lex's life (if she had family besides Great Aunt Beaulah Violet, how she and Max first got together and her early years at Shadow of the Bat), figure out if Lex's boss, Crystal, was meant to be Chrys' mother, how Damion and Isabella lived before Lex's next reincarnation, Isabella's youth, Lyndi's love life before Delchi, how the girls came to be friends, and Sarah's life in the alternate universe before finding her way into the regular timeline.