Of course, Rinnegan was born indeed, but as a complete power that it is, it could do both, destroy and create. It is quite obvious that this time, it's purpose is destruction. Not to mention that there is even the prophecy, speaking about the savior (well it does seem that Raikage knew about it, since he mentioned Minato as Jiraya's pupil, when naming him savior. He might have heard about it from Jiraya).
We have to remember that there are things that only Hokages knew (beside Tsunad, probably ). The third even mentions that Itachi after obtaining knowledge about his ancestor, had wisdom on Hokage's level, with seems to imply that Hokage knew a lot about the past as well.
Still, Hashirama might simply answer that village was created to end constant wars and bring peace. When other clans started doing the same, the two most important people in the village had different visions of the future. This probably comes down to the same thing as it was between Older and Younger brother. Madara probably wanted to dominate them all and create a constant state of peace under Konoha's rule, but Hashirama probably wanted everyone to get along. Kishi already showed that were Hashirama searched for cooperation on equal terms, Madara desired total domination (you can see that clearly in Onoki's flashbacks about Madara).
As to why Ninja were born, well So6p desired peace and thus he was teaching others his ways. It wouldn't be strange that Shinobi indeed existed to help build that peace.