I don't tend to review many films here at IMDb, because I run my own review site, Apocalypse Later. However I felt I should chime in on this one as none of the other reviewers seem to have got the point of this film and I didn't want potential viewers to be put off by their comments.
Yes, there are similarities to The Stepfather. Certain scenes are overt homages to it. No, it's not a rip-off or a remake or anything similar.
This is a feminist take on that whole genre of horror movies. The strong characters are the women, all of them. The men are only physically strong, but otherwise weak. That the 'stepfather' character gets ludicrous towards the end isn't weak writing. It's making a point. The only man in the picture with any strength is the one who's too old to do much that's physical.
There's also a blurring of the roles of 'killer' and 'victim'. Traditionally, Aden would be just the killer, but it's shown that he's a victim too, not to elicit any sympathy from us but to highlight that his failings are because he's not strong enough to be anything else. He can't break the cycle. Traditionally, Cassie would be the victim but she's anything but here. Even when she's playing that role for necessity's sake, she's the strong one in the scene. She just can't match Aden physically so she doesn't try.
I'd really like to see what female horror fans have to say about this film, especially those who have been subjected to abuse and found a way out. I'm male, but it seems to me that this would be an empowering film for abused women, far beyond its value as a horror/thriller that doesn't follow convention. I've seen it twice and felt that it played even better the second time through as I picked up on certain details that I'd missed first time round.
