The most evil person I've played is actually one of my favorite characters I have ever made. It was for an Infamous based RP and I thought it would be fun to bring back one of my favorite enemy fractions from the Infamous games which were the Reapers. So I created a character that was the new head of the Reapers who is basically a mix up of a ton of insane and psychotic characters I could think of that I have seen and enjoyed over the years. Like The Joker, Vaas Montenegro, Kreig, Ripper Roo, Mama (Dredd movie), and etc. Put that in one big melting pot and you get:

Scorn

A person who as a child was a shy and timid girl but after one tragic event loses any form of that innocence and became a cold, calculative, psychotic killer. She'll do anything to get her way and nothing will stop her.
 I still enjoy using her from time to time for other RPs since I can change a few things and make her work with them.