The concept of Evil is a social construct. In order to percieve Evil, one has to be self aware and in a group of other self aware entities. Without a social framework and self awareness, there is no evil, only survival. No other creature on this planet has arguements over the nature of evil, indeed, they cannot be Evil. Fish singularly fail to be evil, but we assign the trait to sharks because they attacks (rarely) humans and cause injury. A shark is not Evil, but the same actions taken by a human would undoubtedly be so.
That said, there is much debate over the nature of what is Evil. Morality is often the word used to define one's ability to discern right from wrong. We would like to think that morality is our own construct, that each of us carefully builds our checklist of right and wrong. This, however, is not entirely true. Nature, or evolution, has given us a rudimentary moral compass, embedded within the very chemistry of our brains. Basically, nature has attempted to ensure that the social community is protected in a way that ensures the survival of the species. This is accomplished by providing a chemical high when we perform actions that help or assist others, and a chemical low when we injure or harm others. This, of course, varies from individual to individual. This innate moral compass interfers with our ability to accuratly define evil, because of the chemical mood adjustments that occur when we think of certain situations. Example:
1) You are sitting in the control booth of a major rail yard. Your instrument panel indicates that a train full of commuters is out of control and heading for an impact within the yard. You can avert the disaster by switching the train onto a clear siding. However, you know that there are at least ten men working on that siding that with most likely be killed if you switch the oncoming train. What do you do?
2) You are at the controls of a siding switch when you notice an oncoming passenger train obviously out of control. You know that you can switch the train onto the siding and save the train, but your work crew is currently on the siding. You have worked with these men for years, your best friend is among them, and you know that they will likely die if you switch the track. You must make your decision now or the oncoming train will surely crash. What do you do?
This question (or one extremely similar) was posed to a number of people as part of a research project. Obviously, the underlying question is the same: do you sacrifice a few to save many? When question 1 was posed, a large majority of people chose to switch the train to the siding and kill the workers. When question 2 was posed, less that half of the same people chose to reroute the train. In question 1, you are distanced from the problem, and see it abstractly, thereby allowing the thinking portion of the brain to analyze and reach a conclusion. Question 2, however, places the action directly in your hands, with immediate consequences to others. This activates the chemical compass and causes clouded judgement. The same process could be applied to the 'kill 100 street urchins to save the city' arguement. As long as you don't personally have to kill the urchins, their deaths seem like the logical course, the greater good. But looking into thier puppy dog eyes and hearing the screams of pain as you wade methodically through them, axe rising and falling, creates a different view point. The underlying action is the same, but the point of view dramatically alters the perception of the event in terms of evil.
I know that this does not answer the question posed by this thread, but I hope it does illuminate some of the difficulty in answering the question. Any situation that causes harm for the greater good can look like a correct and moral decision or a heinous and evil decision, depending on the point of view of the opinion maker. Thank nature for this wonderful dilemma.