Evil is knowingly and willfully doing harm to another, the more harm is done, the more willfully it is done, the more evil the action is.
Now, complicating the issue is that evil actions are often done to serve good purposes, so INTENT is a critical factor.
Sometimes people get themselvee screwed up into believing that killing an individual would be good for humanity in one way or another, and follow through. That person is less evil than someone who kills another they know to be innocent, or for selfish reasons.
However, as soon as you start getting into "end justifies the means" type thinking the whole subject becomes a quagmire. Historys most vicious evildoers nearly all believed they were doing good, from the holocaust to the crusades to the intifada to the communist purges.
All you have is your own reason and conscience, and unfortunately for all too many people one or the other is poorly developed. So societies tend to create mythologies which reinforce commonly accepted moral codes, which often tend to personify evil as a diety of sorts, or a force let loose in the world, which further complicates and confuses matters.
Evil is largely a human construct which allows civilization to function, and not a very good one if you ask me. In Asia duty serves much the same purpose, I think both concepts are necessary.
ETA
Nobody truly believes there is no right and wrong, that there is no morality, but many people disagree on the nature of the question.
Concepts of good and evil derived strictly from faith are only as strong as that faith, and if that faith is lost (or placed in bad or inappropriate doctrine) so is the person.
Morality derived from reason is subject to the frailties of the individual mind.
Liberalism is the lack of either.