I have yet to formalize a good argument on this point, but simply taking a moment to meditate on the subject will be sure to give some enlightenment.
One way of thinking about it, is that most people incarcerated for "evil" acts, example such as murder, was usually, if not always, pushed by ignorance (childhood trauma, parental ignorance, neglect) to that stage where murder was an option without anything being a threat to their life.
Always think of the source! Not the symptom.
I think the purpose of evil is to give every aspect of the world a good and bad. Creation and destruction are natural elements of every part of the universe, so it only makes sense that human nature, even as intangible as it might seem, would have more than one aspect to it. Since there is evil in the world, it allows us to be even more appreciative of the honesty and earnest nature that lies in the hearts of many more than the darkness.
Evil (Or rather negative occurrences/acts ) are inadvertently a part of the balance of life. Human beings are complex but are driven by systems of learning, reinforcement and external affect. The negativity of life drives us to work or push for the positive, exercises our instinctive will to survive.
In many ways, negative acts are viewed as evil now, when once these acts were not considered such a way and were a part of animal life e.g. killing, aggressive acts, sexual relations without limits etc. In the advancement of human awareness, each culture has developed specific views on right and wrong. There are many steadfast rules, such as that of killing which is seen as a crime.
Concept of evil is also driven by perspective . The "right" to commit an evil act is seen as justified if the person is deemed "evil". Killing is also seen as justified when it saves the lives of one or more people.
It is therefore difficult to define "evil" but negative events are a requirement. Too much safety, too much good would result in a life without struggle. Life without struggle is life without achievement.
A life without achievement? I'm sure you can figure out the rest...
I personally believe that evil does have a purpose. If our world existed without evil everything would be a prime Utopian for the most part. If people are not given challenges in life or presented with difficult circumstances they never grow or learn anything new, therefore evil is present to teach and help people grow as human beings.
We as humans, have developed our own sense of morality and this has spread and differs throughout cultures. Evil exists in our world, we can't deny there is both moral and natural evil all around us, but it makes us appreciate the good. Without it our society would be very different, we would most probably be selfish pigs. Natural disasters are a good example, it brings not just the community together, but the world. The Philippines are suffering from a terrible natural disaster recently, which has brought out the best in millions of people from all over the world, pooling together their money and running charity events to raise funds for aid. If it weren't for this evil we would have little compassion as we wouldn't appreciate what we have. We can't live our lives in a protective bubble, we have to accept there is evil and we should embrace it and allow it to shape our characters.
My supporting argument can be explained best by reading a Clockwork Orange by Anthony Burgess. In a nutshell, being evil is as natural to humans as being good. There needs to be both in the world as balance because one is nothing without the other.
Furthermore, as sad as it is, and the existentialists would scream out, everything has a purpose, even tragedies.
Yes, I have read about the balance of the universe, but in my opinion, there is no purpose for evil. A person walks into a school and shoots little children, teenagers, and teachers. What purpose was there behind that? If we had no evil, there would not be a use for jail, or the court system. We weren't born to be evil.
Evil exists and there is no way to entirely squelch it, but it does not serve a purpose, it only harms. The evil ones serve to harm, but that is not the same as a purpose. Evil is a condition, an unfortunate condition, that some are so inclined to at the expense of others and themselves.