So, he literally cuts off the girl's ass...
...and yet, somehow, Noa seems no worse for ware. She's able to walk freely, dance, work-out, and even sit down. All without the least show of discomfort, or time to properly heal.
I do recall some painkillers being mentioned, which may have helped with the agony, but she still wouldn't be doing any kind of physical activity for a very long time. Not to mention the fact that the doses of painkillers she would have required would have rendered her completely useless in forming and executing any type of escape plan.
My only thought is that filmmakers must have just thrown in the ass removal for shock value, and to pit the audience against Steve even more. Unfortunately, they completely failed to follow up on it in any meaningful way.
While I've heard this movie is fairly well regarded, I just found it to be so silly and poorly written throughout. The ass removal may have been a particularly egregious example, though it was far from the only one.
I was actually looking forward to this movie and hoping for something better, but as is, I couldn't rate it above a 4/10.