Mist Body Experiments
As we know, there are still mysteries involved with the Bloody Mist.
One of these mysteries is the fact that there were bodies being experimented on, according to Zetsu
This foreshadows that Rin herself, could had been someone who was experimented on from the Mist, since the Bloody Mist treat her as a thing and "it", as well as Konoha the enemy. They were simply trying to get their hands on their own experiment.
It is certain Kakashi killed Rin. But Kakashi killed Rin out of love. He is a great analyzer and knew that the best outcome in that moment was for Rin not to be captured. He had no choice but to kill her despite doing it out of love.
Kind of how Itachi is a great analyzer and knew the best outcome for the village and peace was to kill his own brothermen and parents.
Now it doesn't stop there, I have a hunch that another experimental body was yondaime Mizukage, Yagura
Yagura seems like another experimental person based on his appearance. And the fact that he had control over the Bijuu as stated by Danzo.
This could resemble Gaara and his relationship with his Dad, because Gaaras Dad treated Gaara as an experiment to control the Bijuu like Yagura.
Anyways as for the motives for the bloody mist, a lot of mysteries are still present, but I think they will reveal more information regarding Rin and such later on.