Man was not created sinful by nature. God created man entirely holy and sinless. Our sinful condition is not the result of God’s creative work, but is a historical consequence of our abuse of free will. To say that God should have created us "for the most part holy and not sinful" fails to do justice to the true freedom and consequences of free will.
Free will means that even though our first parents were entirely sinless and enjoyed the life of original grace, they were still capable of choosing to be otherwise and forfeiting of the life of grace—and that’s just what they did. We, their offspring, are conceived and born bereft of this life, spiritually dead, in original sin. Even when we are spiritually reborn in baptism, we remain affected by concupiscence or tendencies toward sin as a result of having been conceived in original sin.
However, all of this is the result not of some defect in God’s creative work, but of the self-chosen circumstances into which our race fell in the very beginning.