Plot explanation, in my opinion (SPOILER)
People having so much trouble understanding this movie makes no sense to me.
This movie is extremely straightforward.
Gina's doppelganger, after killing the real Gina, gets into the car crash.
Due to the brain trauma, her memory is screwed up & she doesn't recall who she is or what's transpired. She is under the mistaken impression that she is the real Gina & witnessing an invasion (which she is in fact witnessing, but hasn't yet realized that she's part of that invasion & already killed off her original).
At the very beginning of this movie, the real Gina is looking at an x-ray of a patient, when she points out that the heart is on the wrong side of the body - which is a "rare condition". She's in fact looking at an x-ray of one of the invaders (they're our mirror image).
When the doppelganger Gina gets tests done regarding her "delusions", they suggest Capgras delusion (or Capgras syndrome), a disorder in which a person holds a delusional belief that a friend, spouse or other close family member, has been replaced by an identical-looking impostor. The doctor goes on to say that it is very rare, "100 confirmed cases since 1923, 80 in the last 10 years".
Now. If there was only 20 confirmed cases until the past 10 years, when the rate of the cases of these "delusions" skyrocketed, wouldn't that suggest the invasion is increasing quite rapidly & people are catching on more & more?
Anyway, once she makes it back to her apartment finally & realizes she indeed won the fight, she came from the mirror & replaced the original, she can relax, she's realized she's safe & no one's after her, & she's part of the so-to-speak revolution. She cried out of relief, not remorse. She never once shows remorse, so I don't know where people are getting that from either.
As for people who don't understand where these evil doubles are coming from (THE MIRROR) & why they're doing it (did you pay attention to those brief shots from "inside" the mirror? Since it's such a dark, creepy, horrid place in there anyway, why wouldn't they want to come through, take over & live our lives? & if they're mirroring yet opposite images of us, of course they're going to be evil because this movie is based on normal, well-meaning individuals so our mirror double will obviously be the reverse: malicious. I'm sure in the mirror world, evil people have nice doppelgangers. & who knows, maybe the movie is playing with the concept that our inner evils we repress will one day manifest themselves like they did literally in this film. But I digress). Just try to think about it for yourselves, look at all the stylistic devices used; foreshadowing, symbols, etc. & it becomes pretty clear the general statement Ellis is trying to make, then he's left it open to interpretation from that point on & it's our job as the viewer to speculate & use our brains to form our own conclusions, & that's where we fall short. I'm not saying I know word for word what Ellis meant to say, because no one does. Spelling anything out blatantly is in poor taste & form & it undermines the audience, which he's considered, but most audiences obviously need to be patronized these days.
People who hate this movie hate it because they don't get it & they aren't trying to get it because they're used to the media spoonfeeding them mindless, simple concepts. The minute a bit of intellect is required, so many people cop out. This isn't a horror movie in the sense of a mindless idiot running around cutting people into chunks for no reason, it's a horror movie in a psychological way. People with low IQs need not watch this film because (as the message board confirms) it will fly straight over their heads & take a dump in their hair as it goes.