The Ending

I thought the suddeness of the ending, Doug's face freeze framed over red was an effective conclusion to a violent narrative.

I'd rather be hated for who I am, than loved for who I am not.

I'd say in this case it needed an extra few minutes to show what happened.
Lets take "I Spit on Your Grave" as an example of one that could end abruptly. After Stanley dies the film ends. I wouldn't call it a happy ending for her after the ordeal but she did get her revenge. She was right on her back door, probably returned to the house, maybe finished the book she was there to write, maybe packed up and left. Maybe when she returned to Ney York got counseling. Then maybe like in the sequel to the remake she killed the people who had harmed the fellow victims of the support group.
Hills Have Eyes on the other hand. The survivors are still stranded in the middle of nowhere. Caravan and car destroyed. Radio, probably burned. So what happens? Travel back to the filling station, follow the road until they finally see a car to flag down and rescue them? Or do they perish in the middle of nowhere. I don't think many cars use that road otherwise someone would have pulled over to help them.