Aristotle answered this question: A good being can only create evil if a greater good results.
For example: The sin of Adam resulted in the incarnation of Christ.
Some theologians believe that the incarnation was planned by God independent of man’s sin, that God would have done it anyway. Among them is
Rupert of Deutz, a Benedictine Monk and theologian in the 12th century whome Pope Benedict XVI spoke about at one of his Wednesday general audiences.
***Like other medieval theologians, Rupert too wondered why the Word of God, the Son of God, was made man. Some, many, answered by explaining the Incarnation of the Word by the urgent need to atone for human sin. Rupert, on the other hand, with a Christocentric vision of salvation history, broadens the perspective, and in a work entitled The Glorification of the Trinity, sustains the position that the Incarnation, the central event of the whole of history was planned from eternity, even independently of human sin, so that the whole creation might praise God the Father and love him as one family gathered round **Christ, the Son of God. *
You can read the short talk about Rupert of Deutz by the Holy Father at
vatican.va/holy_father/benedict_xvi/audiences/2009/documents/hf_ben-xvi_aud_20091209_en.html if you wish, but my point is that it is not universally accepted that Christ came
only because of original sin or
only to atone for original sin.
Some very strong theologians who have had great infulence on the Church believed that God would have become man anyway, simply to unite his family and to provide them with a means to priase him.
-Tim-