I would define evil as being without the capacity for kindness, empathy or love. This means that "bad" things can be done without any remorse, or care for others.

Personally, I also think it is a combination of nature and nurture. I think the way in which people react to things starts with their genes, and is adjusted by their early experiences. So two people can experience the same events in their lives but have different responses. This could be because they have different capacities for stress, or because they have a different ability to empathise, or a differnt perception of things. It could be a greater or lesser ability to handle emotions. Much of this is, I believe, shaped in early childhood. But I think there is a genetic basis to it. Then, if traumatic events happen throughout the person's life, it CAN, depending on the other factors, "push them over the edge" so to speak. This is why abuse CAN cause abuse, but doesn't necessarily.

This is such an interesting topic.