idk if any1 posted this yet but i think obito unlock his ms after seeing Rin die. the previous chapter said that kakashi and rin were attacked by mist ninjas. my theory is that obito went to the scene and was gonna help but had a change of heart. I dont think kakashi let Rin die, but rather she might have sacrafice herself in order to save kakashi. Obito sees this happening(MS unlocks atm) and his heart is twisted just like Nagato. He changes from good to bad and thus wanting an infinite tsukoyomi...