Been watching and thinking 4 a week now so figured i should probably at least post a general idea so here goes what im thinkin atm:
Definitely want to do a little girl maybe taking the dark creepy look like FEAR/Jap Horror Movies but also thinking i might go the happy cheerful looking cute little girl but with like a bloody knife/sword. Definitely a calm killer but havent got much further than that
if you can
check out the lil girls from the game bioshock, from what ive seen/heard they're cute looking lil girls that steal "powers" from people with a giant syringe. i havent actually played it yet btw.
you should watch the japense version of the grudge!
you could go with a delusional mother who lost her kids due to police carelessness, and now she believes all criminals are her "brood" and tries to "protect" them.
just some ideas