the ending

Hey all, just saw the movie and came here to read what you people think about the ending. It seems a lot of you complain about how disturbing it was. My reaction when Saiva killed Anja was: Ok, she has done it to survive. They would have taken all the food, lots of suplies and two dogs. Now she will kill Loki too and live happily ever after alone. And then I saw her kutting Anja's face off.....and was like WHAT?? It just did not fit into the picture of this character. The ending completely ruined the film for me, the pchyco note was entirely out of place. So illogical and sick, not disturbing but plain stupid.
After all, that is just my opinion. What do you guys think? Have I missed some signs showing her madness throw the movie? What is your logical explanation for her actions at the end?