The Zoroastrians believe that we were neither good nor bad until we had will and knowledge. Once we had will and knowledge we made decisions and the essential duality of nature became human nature.
I think that sounds pretty reasonable, although I'm not a Zoroastrian. This actually accords pretty much with your conclusion: we are both good and bad, depending on the the circumstance, and the person.
A question without an answer is not one I usually think about. 😄
Don't really know about Zoroastrians but i believe they are correct.