why would oru not kill nagato in the start? he was a threat always? he had enormous potential to see the tablets and techniques. oru had met nagato when he had the rinnegan. even though it was deactivated couldn't he sense the powerful chakra?maybe he did and insisted to kill him because he wanted that chakra.so he leaves konoha .so he finds that chakra and needs a dojustu like the sharingan to fight it. so he needs a good alive body and eyes to use ancient techs to fight nagato in the akatsuki.