so the motivation for *spoilers* ...
the motivation to kill Mark the stepdad was that... he's kind of a jerk ? Or was there something darker implied which I missed, like abuse or assault?
Otherwise it is just that the girls were empty and bored, and slightly annoyed, and took things all the way to murder. Which, that is a worthwhile story... I just couldn't tell if Lily saw Mark as such an unbearable obstacle to her life, or was she just prone to suggestion when Amanda casually floated the idea?