Ok while thinking about the events around Rin's death I had an idea. I don't think Rin was working for the Mist Shinobi. She's been built up as too much of a nice and good natured person for her to betray her friends and village. My assumption would be the Mist villages wanted to take her for something. They seemed to be interested in her body, my guess would be she had something powerful in it they wanted. Tailed beast anyone? Those marks on her cheeks kinda look like sealing marks. Just a theory though. Also when you look at her when Kakashi is killing her, there are what look like bubbles around her. These could either be the technique relating to the six tails or could be the forming of a chakra cloak. She also looks a little bit like Utakata. Perhaps she asked Kakashi to kill her if she began to transform or was about to be taken by the mist shin obi. In that case her last words could be seen as a thank you. just a thought though. Interested in hearing other's opinions.