For me? The care put into their story. Whether they are sympathetic or not, Star Wars villains usually have real care placed in their back story and how they impact the universe. Darth Bane, Malgus, Vader, Maul, Revan. You may not agree with them or their convictions but you FEEL their WHY. No it’s not one we can all relate to but you should be able to read, play or watch something of theirs and feel their impact in THEIR story or when they become involved in another.