Thanks for your question.
Adam disobeyed God for the same reason that we do. We have a choice, to obey God or choose that which is evil. Adam made his choice between God and staying with Eve. Eve made the choice to believe God or the lie of the serpent. She fell for the lie. The short answer is sin is our fault. Adam had free will as do we. Adam, as we do, misused that privilege and chose to disobey God.
If God created a world full of evil to begin with then there would be no standard against which to measure sinfulness. We could argue that sin is God's fault and not ours. As it is, the blame falls squarely on us where it belongs. That's why we appreciate God's love and the great Hope He has given us. He is willing to save us from ourselves even though, really, we don't deserve it