The unresolve helps make this movie good, but it's all too unclear

You have to pay close attention to the film. The old woman says she killed Karen and says her daughter was killed on the day of her Communion. Alice never killed anyone. She just started her period and at times it does strange things to young girls. It has a major impact on them.

that ending scene is obvious that alice had malicious motives. i always thought the attack on the aunt was alice.

I agree that the final frame was foreshadowing Alice's future. Remember, one detective read her psychological profile and concluded that she was "nuts". She also lacks her sister's sweet nature and at times seems completely soulless (ie, when she strangles Alphonso's kitten to death).

Yeah, they're dead; they're--all messed up!

Notice Alice's shoes when she drops the milk just before the attack on her aunt -- white and black. Tredoni's shoes are white as she waits in the stairway. Alice does have white shoes in her basement suitcase which act as a red herring, as does Tredoni wearing the same mask and coat (though all of the church goers appear to have these coats) as Alice. I believe Tredoni intentionally replicated Alice's appearance while killing in order to frame Alice, as she views Alice as a child of sin and a "nasty child" when Alice sneaks up on her in the beginning.

The attack on the aunt was likely a mistake on Tredoni's part, as sisters Annie and Catherine look very much alike. Tredoni intended to attack Catherine disguised as Alice so that Catherine might blame Alice as she would recognize Alice's mask. I think the intended Catherine attack was not intended to be fatal for this reason. She wanted the daughter Catherine conceived out of wedlock to be both shipped away to be locked up, and for Catherine to realize the sin she committed is haunting her; in reference to Tredoni's later line about parents and sin.

Now, as for Alice: Alice is clearly a psychologically disturbed young girl as evident by the number of professionals who call her nuts -- one even indicating a Schizoid Personality Disorder and another who is baffled that she truly believes Karen attacked her aunt (it's hinted that this state of mind is brought on by Alfonso's warning that Karen will come back for her doll; Alice then encounters Tredoni in the basement and arrives at this disturbed conclusion). Alice is both lacking emotion and experiencing far too much throughout points in the film, often simultaneously, which makes her the obvious suspect. She is not the murderer in this film (though we are given many indications this is the path she may one day follow, especially after the trauma she undergoes here), the confusion is created by director Alfred Sole's brilliant use of irony, incredible coincidence, and for much of the film, ambiguity. What I took from the final shot is that Alice, after the death of Father Tom, is at once more disturbed than she has ever been, but also eerily fascinated by what she has just witnessed. Her clutching of Tredoni's bag and knife were, to me, her newest items among her suitcase of unsettling memorabilia.

~~the first rule of fight club is have fun and be urself~~

To be fair (to Alice) she doesn't have a "vicious little grin" on her face at the end of the film. She's not even smirking. She has an almost totally blank stare on her face. Her actions in the last shot are obviously intended to leave the viewers wondering if she will turn out to be a psychopath, but it's inaccurate to say that she looks happy or pleased with what just happened. She looks stunned and contemplative more than anything.

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Alice is seriously unbalanced and has an unhealthy tendency to "fetishize" things. That's what her little collection of things in the basement is about. Her sister's stolen doll, etc. So when she steals the knife, I take it that she's adding it to her sick-minded little "collection" of objects... which is kind of like the Catholic church's veneration of "relics."

O' course, that's just a theory, but it's my interpretation. She may or may not go on to become a killer... but, she probably won't. She is, however, crazy as hell.