Evil is the act of performing actions with little concern for how these acts will (negatively) impact others; typically for the personal gratification of ourselves. As you can see the source of evil would be within ourselves, and could be argued to be following our more basic animal instincts.
While I don’t prescribe to a utilitarian worldview, the general consensus on evil seems to be that the more harm is done to another individual the more evil an act is; and the less gain an individual had for performing that act the more evil it is. To put this into an example, torturing an individual to gain important information from someone is (generally speaking) less evil than torturing someone because it is amusing to you; assuming the harm from the torture is equivalent in both cases.
