I notice in a lot of media there is a strong emphasis on the greatness of evil characters. They can often end up more popular than the good guys in the book or on the show, and overshadow everyone else around them.
Darth Vader is a good example. He is a despicable human being who has committed genocide and is responsible for the deaths of billions of people. Yet the common talk is of how much of a badass he is. His coolness has made him one of the most popular characters in history. He is on lunchboxes and has action figures, and people dress up as him in costumes.
How can you write an evil character while reminding the reader that he leaves behind real damage? That he causes suffering and pain to flesh and bone people without dwelling on how awesome he is while doing it?