While watching the movie, especially during the parking lot scene, the shot gets switched from POV (which is unusual since most of the movie is shot in POV), I convinced myself that the shot is switched from the POV 'cause the victim (a lady) is out of sight from Frank and director wanted the audience to show where the victim is and how she's terrified of Frank.
However, after reading your question, I did some search on internet and found this interview with Franck Khalfoun (director of the movie). He explains the reason why the shot is switched from POV in the parking lot scene.
How did you choose those spare moments where we do see him?
KHALFOUN: I needed to see him at some point. It’s hard to sustain not seeing your character through a whole movie so we had to figure out ways for me to see him sometimes. Once you see him you want to see him again soon so you start looking for him throughout the film. One of those was mirrors and reflections, obviously if you’re looking in a mirror you see yourself so if you’re in a guy’s brain you can see him see himself. Another one was dreams, when you’re dreaming you see yourself a lot of times in your dreams so that conceptually stayed within the framework of POV. Then in reading and doing research I saw that a lot of these guys, a lot of these killers, have out of body experiences where they aren’t in themselves, they’re being pulled out and watching themselves kill from the outside. So I thought that’s interesting, we’re still within him but were experiencing this out of body thing. So that’s why it pulls out.
That’s the shot I was really curious about, the one in the parking lot.
KHALFOUN: Where he’s killing and all the sudden we pull out and you see him doing his thing. He’s experiencing watching himself. He’s having an out of body experience while committing this horrific act.
That is very cool.
KHALFOUN: It is cool and it allows us the audience to participate an experience that serial killers have talked about, and it also allows us to see the character as he commits his crime.
Since you're very much intrigued about this movie, try this interview with Franck Khalfoun.