It's a question with a complex answer. From the collective human perspective, evil is wrongdoing, an action which causes harm to other sentient beings (just like Octavia explained it). But it's also important to note that, just like Thparky said, nobody does a wrongdoing for the sake of doing wrongdoing. Every wrongdoer has a justification behind their actions. No wrongdoing is done simply because somebody is 'evil'. Therefore, from the individual human perspective, evil does not exist. At least not in the absolute or complete sense.
And now, here's my piece of the puzzle. Evil can be present only among humans. Sun gives you light and warmth, but it can also burn you and give you skin cancer. Is it good or evil? Water is essential for all life, but you can also drown in it. Is it good or evil? Predators viciously hunt and kill other animals, sometimes including humans, but they need to do it to survive and they regulate the population of the species they prey upon. Are they good or evil? What I want to say is that, in Nature, evil does not exist. And similarly, nothing and nobody is born or inherently evil.
One can only become evil. One way to become evil is to experience evil. In the words of Carl G. Jung: "The healthy man does not torture others - generally it is the tortured who turn into torturers." Another way is Ego and all the vices that derive from it (greed, selfishness, apathy, jealousy, envy etc). There are other ways, as well, but nothing is evil in it's Nature, not even humans.
