End of the movie is a plot hole, and it sucks (spoilers)
I watched REC and REC2 back to back just now, and I have to say that this ending doesn't make sense.
The priest quotes the Bible, saying, "The Lord keeps them in eternal prisons of darkness until the final judgment day," which they explain means that the Medeiros girl can't live in the light because she is directly possessed by the demon. "Where she is, light is not reflected." This is a property of the demon, not of Medeiros. By any sort of logic, when she transferred the demon to Angela, Medeiros would become visible since she was just a human once, and Angela, who is now the demon, should be invisible. Also, with respect to the first movie, Medeiros shouldn't have been able to attack Pablo the cameraman and break his light if she doesn't exist where light is.
It just ended in a horrible way. If things existed in total darkness, there could easily be a way out of that building besides perfectly mimicking the priest's voice anyway. In fact, it kind of sucked throughout; we've got a SWAT team that is seemingly incapable of combat (or shooting things in the head), three annoying teenagers running around, killing a possessed woman (careful not to say infected) by sticking a bottle rocket down her throat, and a ridiculous ending...gah.
I've seen all three now, and I have to say I enjoyed the third one the most. And I'm so glad it didn't continue with the stupid ending on this one, but rather with the dog at the vets.