A plot twist ending that I feel would have worked...

I remember feeling like it was a twist as is, because while watching I thought that it was going to turn out as you've described. I wouldn't have minded if they had gone that way, but I wasn't disappointed, as is.

Also, even in a grim and disturbing movie such as this, don't we nearly always want a happy ending or at least a form of catharsis with the perpetrator getting some comeuppance and not just getting away?