A Day in a Perfect Life
For the sake of illustration, lets imagine that there were multiple perfect things that you could engage in. Now imagine waking up in your perfect home (which is identical to everyone else's perfect home), surrounded by your perfect family, who loves you not because they have a choice, but because that's the only thing they can do - after all, they're perfect too. You get out of your perfect bed and have the perfect breakfast, followed by the most utterly perfect mid-day activities and family time. You wow as you enjoy the perfect picnic at the perfect park, your taste-buds burst in ecstasy as you consume the perfect dinner with your perfect family. After your dinner, you engage in the most remarkably perfect sex in the most perfect position with your perfect wife, and at night, you and your family all lie together in the meadow, looking up at the perfect stars in the perfect night sky - each of you has gone through the entire day in an unshakable and unrelenting mental state of ecstatic loving bliss.
Now imagine that every single family on earth is doing the exact same thing that you're doing, perhaps at different times (depending on geographic location), and all without any choice to deviate from this objective 'perfect' state of being. Imagine that every day will be exactly the same and that you will never die, nor will you ever deviate from this state of existence.
'Perfection' Eliminates the Purpose of Being and Free Will
Besides being terrifying to imagine, a perfect world, with perfect beings, and a perfect material nature cannot accommodate purpose, actual freedom, or a reason for being. In a perfect world, there are no problems to solve, no diseases to cure, no triumphs, no struggles, no tears, and no victories. In such a world, we would simply exist as choice-less beings with no objective meaningful purpose or means of defining our own existence.
Bad Defines Good and Good Defines Bad
Without bad to define good, good is nothing. When you see a beautiful sky on a sunny day, you can appreciate it because you've seen stormy skies that bring rain and destruction. In the same sort of way, when you love deeply, you can appreciate it, because you've known what it means to feel alone.
A perfect world is like a perfectly white room in which you, and everything around you are also perfectly white. In this room, you see nothing, because there is no contrast to define one thing from another. The same would be true of a perfectly black room. It is only by allowing the freedom of both elements, black and white, that innumerable variations of grey can lend form and meaning to this room and all that lies within it.
So in our perfect world scenario, you can't really love or really enjoy anything, because you have nothing to define what makes anything worthy of love or enjoyment in the first place. Everything you do, and everything that happens around you, is simply a set of incomprehensible actions that take place without anyone ever having chosen to do them.
The Scope of Good and Evil
Since good and evil are two parts of an integral whole and necessary to define one another, we must consider that the greatest possible experiences of love, joy, or attainment exist in polarized contrast to equally powerful capacities for hate, sorrow, and failure. We, our world, and the universe it occupies are free, and such freedom requires both good and evil to exist as unrestricted elements. To limit the scope of how bad things could be, would be to limit scope of how good they could be also, ultimately this would diminish our own freedom and depth of experience.
Because Evil Exists, We Have Purpose and Meaning
As humans, it is our transcendent purpose to choose good over evil. When we choose to engage in things like love, friendship, creativity, caring for our fellow beings, or improving the world around us, we find unison with God, and we feel the transcendent beauty, good, and meaning of it.