First we must define what entails evil. That is, does someone who is generally viewed as "evil" view him or herself as evil? Did Hitler believe himself to be evil? I think perhaps, more likely, he thought himself a positive force to the world, for doing it the favor of eliminating the undesirable elements of the human gene pool.
Perspective, therefore, makes evil just as impossible to identify as good. We may throw the word evil around, but what does it really mean? Who are we to judge this? If you believe in some sort of religion, you would leave it up to your primary deity.
For the sake of the argument, though, let's look at a small act of evil. Say someone has done something that has really hurt me. Let's say, for example, that you're dating someone and this person, while still involved with you, sleeps with someone else. You discover this. Now, how do you react? Let's say that you act very spitefully towards your former partner: yelling, berating him/her, damaging property, even acts of physical violence. Is this qualified as evil? Perhaps, on a small scale, it is. Is this your natural reaction? Yes, I believe it is. However, there is also the nurtured part of the human psyche that allows most rational human beings to decide what is correct and proper, and allows them to avoid acts of evil on a large scale.
Therefore, evil at its purest, such as Hitler or Stalin levels, is neither natural nor nurtured, but caused by some irrational mental disorder. Sadism amplified to a horrible magnitude.
"Evil" is not how it is painted in the movies. Even if you point to traumatic events in childhood, evil is not justified. It is caused by insanity and the loss of the ability to distinguish between right and wrong.
What about larger scale acts of evil? Terrorism, for instance. Osama bin Laden, you may argue, is evil because he has lost his ability to recognize the horror of his acts in relation to humanity. But what about the underling terrorists? Are they, too, evil, because they carry out his orders? Or are they simply pawns, brainwashed, unwilling to admit that they are doing something "evil," but aware of it at the same time? The Nuremburg Laws would say that, yes, they are accountable. But are they beyond correction, to the point of Hitler?
The question of nature vs. nurture has also been applied to the subject of homosexuality, and I don't think there has been any resolution there. Psychologists & philosophers will debate into eternity.