My two cents on the ending

The ending just doesn't make sense. If Adam isn't guilty why throw in the woman who claims she was raped by him? She'd be relevant if he was the killer.
It also doesn't seem logical for a brother and sister to willingly have sex with each other. I think the idea of a sister in love with her brother in this film was meant mainly to be a shocker to the viewers, but it just ruins it.

A few plots I think would make sense:

PLOT 1
The way I see it, the whole ending would look much better if it turned out that he raped his sister, thuse proving he was a violent man and she decided to revenge him by killing every woman who made him happy, to destroy his happiness.

PLOT 2
In my opinion, things would also make much more sense if he truly was a violent man, who could beat up a mugger to a near death, rape his sister and another woman and kill two of his lovers. That would be a scenario I could accept.

PLOT 3
By far the best plot I came up with is if towards the end they made us doubt Adam is the killer, by dropping the guilt on the sister and then have Heather shoot the sister, just to discover Adam really was the killer as first suspected. That would really be shocking and deeper ending because it would make the viewers wonder if the sister didn't take on the blame for her brother's killing's in order to protect him (which would make her a partner in crime anyway). I think it's the best ending for this film because the way I see it, the ending deals with the way the sister loved her brother. It could present her love in a more logical way, that would make much sense to the viewers and also be kind of a shocker to them as well.

Feel free to agree or disagree with me. It still makes much more sense than the original ending of this film.