People keep telling me to go play GTA V. A sandbox game. Not an RPG. One which follows a strict, linear story ending in three different choices. I think people are confusing gameplay evil (like taking an AK 47 and just going on a shooting spree) and RPG evil. (ie choices which you the player can make which affects the story). Bioware used to be so good about this.
These are RPGS after all. They include stuff like binding a suit of armor to a man's soul and forcing him to do our bidding while enslaving the minds of our own party members to forcing a wookie to kill his best friend. And then we have the the lackluster choices made in Mass Effect 3 which don't really amount to anything.
No offense but you can't really tell a "story" with evil, evil is just a symptom of abject personal failure, the final line of a story that ended a long time ago. It doesn't have any diemensions, narrative, emotional, physical, etc.
Heroism is the only concept you can iterate and make it increasingly intricate and powerful.
Rampantly slaughtering people in GTA is about as close as you'll ever get, you want someway of making "evil" "good" and I hate to be the bearer of bad news but that's not going to happen.
Even all evil playthroughs in KOTOR, SWTOR, ME, are rife with competing choices and outcomes, weighing and balancing, that make them less than the pure evil you desire.