From a roundup of reflections to mark the 10th anniversary of 9/11, the one by Terry Eagleton on evil provides an opportunity to identify a key difficulty in the philosophical definition of that concept. The definition he urges upon us can be seen from this passage:
What happened on both occasions [September 11 1973 in Chile and September 11 2001 in the US] was a moral obscenity and wicked, but it was not, in a technical sense, evil. There is a distinction between evil and wickedness. It is wicked to destroy innocent people for one's political ends, as Al Qaeda did that day in New York and the United States has done in Vietnam, Cambodia, Afghanistan, Iraq, and countless other places around the world. For an act to be evil, however, means that the destruction must be done simply for the hell of it - for the sheer obscene pleasure of the thing, rather than for some functional end.
It follows from this that, for example, someone's torturing an animal to death merely for pleasure might count as evil whereas their torturing hundreds of people to death in order to intimidate a population threatening the oppressive regime they work for wouldn't. And a short and obvious answer to this proposed definitional restriction is: pull the other one. To be persuasive a definition needs to capture the core of our intuitions on how the relevant concept functions, and the suggestion that torturing large numbers of people to death isn't evil provided only there's some end in view would be widely rejected, since extreme cruelty to sentient beings is one of the paradigm meanings of the word 'evil'.
In my experience the same sort of move defeats many philosophical definitions of evil that purport to delineate the concept by treating it as a sub-domain of more generally wicked acts: evil are (only) those very bad acts done without conscience, or very bad acts done for sadistic pleasure, or very bad acts that are meant to deny a person's humanity; and so forth. You can always find a counter-example in which some monstrous harm is perpetrated but with a qualm of conscience, or without pleasure but with indifference, or not to deny but to emphasize the victim's humanity, by making it indeed a torment to him or her - and come up with the enduring sense that these acts as well are evil.
I go no further, for the time being, than registering the problem.