I don't think Annie shot John, I think it wouldve been very clear if this had happened. Given the structure of the house and the sequence of events, it makes more sense to me that John simply jumped out the Attic window and landed on the Entrance pavement, killing himself (out of dread, survivor's guilt, or whatnot).
I believe what Annie was thinking was that the curse spreads when they die by the curse via the "thunderclap", so if she kills herself before that occurs then the curse can no longer spread. For one reason or another (perhaps she was just too late, or perhaps this only makes the curse proceed), this didn't really work.