But, for the sake of argument, let's continue. Let us suppose that this perfect God did create the universe. Humans were the crown of his creation, since they were created in God's image and have the ability to make decisions. However, these humans spoiled the original perfection by choosing to disobey God.
What!? If something is perfect, nothing imperfect can come from it. Someone once said that bad fruit cannot come from a good tree, and yet this "perfect" God created a "perfect" universe which was rendered imperfect by the "perfect" humans. The ultimate source of imperfection is God. What is perfect cannot become imperfect, so humans must have been created imperfect. What is perfect cannot create anything imperfect, so God must be imperfect to have created these imperfect humans. A perfect God who creates imperfect humans is impossible.
The Christians' objection to this argument involves freewill. They say that a being must have freewill to be happy. The omnibenevolent God did not wish to create robots, so he gave humans freewill to enable them to experience love and happiness. But the humans used this freewill to choose evil, and introduced imperfection into God's originally perfect universe. God had no control over this decision, so the blame for our imperfect universe is on the humans, not God.
Here is why the argument is weak. First, if God is omnipotent, then the assumption that freewill is necessary for happiness is false. If God could make it a rule that only beings with freewill may experience happiness, then he could just as easily have made it a rule that only robots may experience happiness. The latter option is clearly superior, since perfect robots will never make decisions which could render them or their creator unhappy, whereas beings with freewill could. A perfect and omnipotent God who creates beings capable of ruining their own happiness is impossible.
Second, even if we were to allow the necessity of freewill for happiness, God could have created humans with freewill who did not have the ability to choose evil, but to choose between several good options.
Third, God supposedly has freewill, and yet he does not make imperfect decisions. If humans are miniature images of God, our decisions should likewise be perfect. Also, the occupants of heaven, who presumably must have freewill to be happy, will never use that freewill to make imperfect decisions. Why would the originally perfect humans do differently?
The point remains: the presence of imperfections in the universe disproves the supposed perfection of its creator.