Good question.Under his villages orders, he was sent to assassinate Hashirama - the First Hokage. He failed but survive and was able to make it all the way back to his village.
Is that even possible? :shrug:
Like EMS Madara which is one of the most powerful characters in the whole series if not THE most powerful character couldn't defeat Hashirama and almost died even when he had EMS, PS, Kyuubi etc.
How did Kakuzu survive - especially in regards to the fact he still didn't have his Five Hearts and lost to Early pt. 2 Kakashi and Naruto?
Hashirama knows grace and mercy. He even likes the clan that gave him the most troubles...the Uchiha.
So I suppose he spared Kakuzu's life.