Calling someone "evil" accuses the person of acting in bad faith and alleges dishonest ulterior motives beyond pure egoism. Through these negative connotations, this falls into the category of AdHominem? attacks. Additionally it demonstrates, that the speaker does not AssumeGoodFaith. Calling someone evil creates a SeparationOfPositions? before a real dialogue has even begun.
"Evil" begets "Evil."
It seemingly inevitable that if you accuse some group or person as being "evil," they will eventually accuse you of being "evil" in return. Since people perceive themselves as acting in GoodFaith, admonitions that they are evil will make them assume you are trying to assassinate their character, which is Machiavellian, and therefore you are evil.
Projections and illusions
People who are uneasy with part of who they are and what they do will more likely project these parts onto the world outside. They do this because it is less fearful in the short term than to internalize the feelings and thoughts and let them change who they are. The evil within becomes the evil out there.
People who accept all the parts of who they are don't have the need to project anything onto the world. They tend to let reality shape them into who and what they need to become. They take the good with the bad, accepting reality as it is: They know TheTruth? and it sets them free. Life is much less a struggle and the world does not look as mean and evil to them because they have much less fear to project on the world. They tend to have faith and a good heart.
Most people who are projecting their own fears onto the world can not and will not accept any alternative view: neutrality is no longer a option for them. They create a illusion of moral superiority: Anything they do is Right. Anyone who does not see the world as they do or does not look the way they do is Wrong and most likely Evil.
When people in authority are projecting their own fears onto others, this breeds hatred. They see the world as a huge never-ending war, reinterpreting peace as a time to prepare for the next war. When they win the war, they will start to project the same fears from a small elite onto some part of themselves. This turns into full blown paranoia very quickly, often resulting in a WikiPedia:Totalitarian government.
EvilIsEvil because the evil you tend to see is more likely a projection of your own fears, uncertainties and doubts then really part of reality. Which does not mean that there is no crime or injustice: Just that behavior does not make a person evil. Justice is about correcting unwanted behavior, restoring peace and making amends for wrong doing; evil has nothing to do with it.