Ok, so here's my theory. Madara knew that obito cannot be swayed that easily unless he experience something traumatic. He needed obito to carry on with his plan coz he was already dying, and obito has the potential to be a great successor. all he needed to do is make obito decide for himself to join him. So what madara did was to set up a scenario that obito would witness and make him change his mind about everything, and that is to see rin die. madara knew that rin means the world to obito, and seeing rin die in the hands of his rival would definitely trigger obito's dark side and make him go nuts. in addition to this, it would awaken obito's MS, making obito his perfect tool. Now this part could be wrong, but if we could all remember, the mizukage was influenced by madara at some point, but wasnt really mentioned when this exactly happened. so im guessing that it was probably that time. Madara influenced the mizukage for an attack to both rin and kakashi. it was mentioned that they were all jounin or anbu level. i gez kakashi was put in a situation where he had no choice but to kill rin by himself. for whatever reason, im sure he had a good reason for that, and he better had, or else kakashi was really a ****ed up guy. So all zetsu had to do is to lead obito to the place where it was all happening, at the perfect moment to see kakashi kill his beloved rin. Boom, obito thirsts for revenge to both kakashi and minato, awakens his MS, and becomes the perfect villain and nightmare to the whole shinobi world. it has been a pretty long thread, but let me know about your thoughts you guys. Thanks!