I think Tom is on point with the gender roles interpretation, but I wanted to expound on it.
I take from this film the idea that, to men, females acting outside of their "female role" prescribed by men are viewed as something alien. I also believe this is somehow mirrored in the aliens' ideas of gender roles, or whatever their equivalent would be. The aliens are somehow selecting a subset of themselves to take on women's skins, and then are seemingly only told enough about the world to complete their task. They know how to drive cars, select lonely/alienated men, flirt with them, and bring back to the house for processing. But it seems that she hasn't been told anything more about her existence; I'm not even sure that she knows in the beginning that the men she brings back are being killed (it's possible death means something else to the aliens as it does to us.) Her male alien (malien?) handler seems to obviously know more about what's going on, and has some way to track the female aliens. They are using the female aliens in a similar way as to how the female aliens use male humans, which is further similar to how men traditionally use women. The ones being used are recognized as being good only for certain pre-selected tasks.
At some point, Scarlet's character recognizes something, either that she is killing humans, or that she is being used by the male aliens, or that there's more to humans than she was originally led to believe. That causes her indifference/repulsion to what she is doing, which makes her break out of her submissive role, and attempt to run away, and figure out if it's possible for her to live as a human. Upon reflection, I loved the ending. I think the trail guide predator guy was a representation of traditional male values in regards to women. When he sees her as simply an attractive woman, all he wants is to use her for sex. But as soon as it's revealed that she is something complex, something that can be learned from, he instead decides to instantly destroy her, to eradicate the idea that women can be something more. I don't think a showdown with the male alien was necessary, as we already know what happens in that case. As we saw with Scarlet's predecessor at the beginning of the film, who gave up and decided to instead lie in the ditch, the female aliens who no longer accomplish their tasks are brought back in, replaced with a new model, and presumably reprocessed. Furthermore, I think the female aliens task to only bring back men reinforces the idea that the aliens understand our gender roles. They want men's skins, to imitate men, as men are the "superior" sex.
One last note: I think her being entranced by the mirror was a reference to the mirror test for self-awareness, traditionally used for animals. She sees herself as a unique identity, the question being, is the identity as the female skin she is wearing, or as the alien underneath? Or does it make a difference? The male alien doesn't do the same thing when he later looks in the mirror, I assume because he hasn't reflected upon the self as she has.
If it's not obvious, I loved this film, without even going into the cinematography or music.