It makes sense. I guess it's what caused them to believe it is normal that puzzles me.
I think some of that is addressed in tlcya's great post a few posts back, which a few people have observed in this discussion. The girls may well have bought into and sold each other a fantasy world, in which violence enabled them to feel powerful and special. I do think at least one of them is mentally not well, and both are certainly capable of cunning.
I think they had a chemistry between them that wasn't healthy, and as a parent who had had to deal with that sort of negative duo stuff, I can say there ARE plenty of red flags, if you're paying attention. Kids who are spiralling into a world of their own give off signals, though on the surface they may look like just BFFs. And they do get super secretive, so it's hard to spot - especially if you're not looking. And if you're paying attention, you can squash it -before- it turns nasty, as I did.
Obsessive friendships do give off a weird vibe, though. I remain truly surprised there was NO sign of this coming anywhere. If that's actually so.
Interest in horror alone isn't unhealthy, as a parent or a child. I don't care what anyone else thinks, I know this because I -live- it, thanks.
There's plenty of sick-minded, callous and violent people out there, kids and adults, who aren't into horror.. so my belief is that the girl's behaviour has its roots somewhere else entirely. Obsessive friendship, perhaps unrecognised illness, perhaps things more mundane (jealousy) or as Mrs. G put in the last thread, perhaps a combination of these things.