The doctrine that God looks after all that he has made and that he uses evil for a good purpose is called the pretentious divine providence.
God can only need evil for a purpose if there is no other way to make us good. This idea cannot be true. or.
God supposedly made the first human beings, Adam and Eve, as perfect. They sinned and the Church says their sin affects us too and we have inherited spiritual damage from them. All he had to do then was to prevent them having children and make another couple to populate the world in their stead so that they would not spread their weaknesses towards sin. Human evil then is useless compared to the good that has been brought out of it. The evil in us could have been easily prevented.
The very fact that evil is that which should not exist, alone proves that God need have no purpose for it.
Even if you have only one sin, that sin defiles your goodness because you won’t give it up. When you do good as you hold on to sin you are saying you will do good on your terms and when it suits you. You hold on to the evil in your heart as you do the good. Thus you turn the good into a counterfeit of good. Considering how most people are "sinners" and have barely any love for God clearly sin is more powerful than holiness.
God had to make us with more good or more evil in us. You are either unholy or holy.
Christians claim that God can’t put goodness in us and we have to do it ourselves. The alternative then is to put less goodness in us or none. If God makes us good we can have the choice of keeping it or losing it. Christianity says that God would be evil if he put perfect good in us. Then they say he is good because he made us less than good. That is clearly saying that God made us evil and is right to even though it is wrong. If he can put evil in us at all he can put good in us and indeed would be better off putting only good in our hearts. The Christians worship a God of evil.