It was primarily because of his waging total war and his vengeful attacks on South Carolina (what happened to Columbia was morally equivalent to the firebombing of Dresden), that I see him as being particularly evil. Some of the names on my list are there because they pioneered profoundly evil doctrines or practices. Sherman was called the first modern general, and I think it's an apt (and not a complimentary) description. YMMV, of course.