There are some darker parts, I guess. Or dark-ish at least. I think so far there is a decent balance between those and the bright ones, tho. But it's probably best if you see for yourself. :)
As for the crime scene not being guarded by the police: That's being answered in the game. You'll find out if you keep going. ;P
If you want to chat more about the story, why don't you join our Discord? There are always people willing to discuss the story and any possible theories about either the murder or the girls' identities. :D