[SPOILERS FOR FILM'S ENDING] Can someone please spoil the ending of the movie The Basement with Mischa Barton for me?

There's a movie called The Basement (2017) on Netflix with Mischa Barton in it. I haven't watched it. I tried to find a plot recap to read but a recap is not available on wikipedia or anywhere else online really. There's a lot of reviews online but they avoid spoilers. What's the twist at the end?

Edit: I don't want to watch the whole movie myself because I don't enjoy torture films and this movie didn't get that many good reviews. I mostly want to know if [spoiler]the guy who gets kidnapped was a serial killer in the past[/spoiler]