one thing I didnt get (spoiler)

I guess you could say it highlights how insane the killer is because he's murdered a little girl. It also involves the police to a greater degree in the story.

It also gets most people out of the sorority house so Mrs. Mac can get killed in the attic.

It adds a little light to what our villain was up to before scaling the house and breaking into the attic

As a few other posters here have mentioned, there is nothing to confirm Billy has anything to do with Janice (the thirteen year old girl) being murdered, nor the rape of the townie that was discussed at the beginning of the movie.

However, listening to the calls Billy makes throughout the run of the movie, a few things stand out that could indicate he is responsible:

* Billy's calls are sexually explicit to a chilling degree, he uses the C word and talks about what he wants to do to the girls. He's clearly perverse and obscene.
* When he calls, the noises and gasping he makes on the other end of the line made me think he sounds as if he may be masturbating, even while he seems to be recalling conversations and events, it sounds like he takes sexual pleasure in his killing and the chaos it brings.
* He makes reference on the phone to telling Agnes not to tell what they did, suggesting he has been abusing his sister. He perhaps likes younger girls (like Janice).
* He plays out Agnes's reactions to his abuse on the line. "No, Billy! No!" Repeatedly. He sounds as if he's enjoying himself during this part...the way a rapist may enjoy the screams and struggle of a victim (to which the townie was raped).
* He plays out the part of his father or step father saying "what your mother and I must know is" with strangled references to where Agnes is....what happened. It seems he killed Agnes...and I think its an indicator he killed the girl in the park (why else have it in the story if for no reason).
* At the beginning of the film he is shown as being outside of the sorority house, suggesting that he does venture outside on occasion, so could definitely be responsible for the crimes. Let us not forget, we aren't privy to his every move.

All those things to me indicate he's meant to be responsible. I think he's not a calculating murderer but a pure absolute lunatic, utter psychopath and dangerous sexual predator. His phone calls seem to indicate he has lost all touch with reality will attack any woman with opportunity, young or old.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We've become a race of peeping toms.

I've always taken the girl's murder in the park to be a sort of red herring. We're meant to believe that they are somehow connected and that there's a serial killer on the loose, while meanwhile the villain is hiding in the sorority attic the entire time.

What if the dead girl in the park was Agnes?

One of the things I like about Black Christmas is it doesn't answer all the questions it raises. We assume Billy killed the girl in the park, but we can't say definitively. That leaves events open to the possibility of a second killer, which adds to the suspense and paranoia. It also makes Peter a solid red herring for much of the movie, further blurring what's really happening. If Black Christmas had the usual wrap-up, answering all the questions, I don't think it would hold all the power it still has all these years later.