God Himself said that man was very good. This is an expression of pleasure; He was pleased with what He had accomplished. If we were very good when He created us, then that must include the nature He created us with.
Does God take pleasure in a nature that is enmity against Him (Romans 8:7)? Not at all, which indicates very strongly that, as He created us and as we are born, we do not have the nature that we later come to have that is enmity against Him. Hateful, human nature is something that develops because God put a spirit in us that other spirits, either God Himself or the spirit of this world, are able to communicate with (I Corinthians 2:10-12).
If the spirit that He is talking about in Genesis 1:31 was very good, then why has human history been a recording of violence, disease, anguish of spirit? Why is there so much bitterness, anger, prejudice, resentment, doubt, self-pity, vanity, envy, greed, jealousy, pride, and lust? Nowhere in God's Word are these called good or even acceptable.
Those traits do not come from God. He did not create us that way. God is love. He is kind, generous, good, merciful. We can honestly conclude that they do not come from man either, as he was created by God, because God judged man to be "very good." Yet, mankind expresses these very attributes.
Again, would something God pronounced "very good" produce what we see? We can conclude that these traits must have come from the spirit of this world, from Satan, who is invisible, soundless, but is able to communicate with our spirits.