Lots of ideas being bandied about, but I'm pretty sure I get what's happened. Major spoilerage, though, so don't read if you haven't already seen the movie - but might.
That said... I'd wager that everyone was already dead at the beginning of the movie. The couple were ghosts who didn't realize they were already dead, thus the girl whispering, "Don't open it" in her sleep near the beginning of the movie and then again at the end just before revealing her corpse. After panning across her corpse at the end, you could see that the chain was hanging off the door - it was already unlocked. That's the way the door was near the beginning, when Alex first opened the door to let David in - he noticed the door had already been unlocked and the chain hanging there, and had to go back into his apartment to retrieve the key so he could lock it again.
David, too, was already a ghost by that point. As was the woman in the apartment upstairs - though I suspect she wasn't one of David's victims, but just another ghost living in the building. It's possible that the playing with the lights _could_ have been the living, who they couldn't see - kind of like in "The Others".
That's my guess, at least. Makes more sense (to me) than some of the other speculations I'd seen. I don't see any parallels to "Paranormal Activity" at all, and in fact would say this movie exceeds the PA franchise by a great deal in terms of quality.
That said... I'd wager that everyone was already dead at the beginning of the movie. The couple were ghosts who didn't realize they were already dead, thus the girl whispering, "Don't open it" in her sleep near the beginning of the movie and then again at the end just before revealing her corpse. After panning across her corpse at the end, you could see that the chain was hanging off the door - it was already unlocked. That's the way the door was near the beginning, when Alex first opened the door to let David in - he noticed the door had already been unlocked and the chain hanging there, and had to go back into his apartment to retrieve the key so he could lock it again.
David, too, was already a ghost by that point. As was the woman in the apartment upstairs - though I suspect she wasn't one of David's victims, but just another ghost living in the building. It's possible that the playing with the lights _could_ have been the living, who they couldn't see - kind of like in "The Others".
That's my guess, at least. Makes more sense (to me) than some of the other speculations I'd seen. I don't see any parallels to "Paranormal Activity" at all, and in fact would say this movie exceeds the PA franchise by a great deal in terms of quality.