This discussion is about the classes,which,in your point of view,are the most evil.
Death Knights (third generation) - majority of them are raised in undeath against their own will. They wield the forces of death and are made for a sole purpose - destruction. They can't live without inflicting pain. As Legion showed us,they are ready to do anything to reach their goals.
Warlocks - wielders of the fel magic,they are the masters of curses,destructive abilities and masters of demons. They are ready to drain somebody's soul,only to empower their magic,they crave for power and are risking to corrupt their very own soul.
Demon Hunters - to destroy the Burning Legion,they willingly became the half demons themselves,forever embracing a demon inside themselves to use it's power to destroy his brethren.Wielders of a fel magic and filled with vengeance,they will do whatever is necessary to destroy the Burning Legion.
Shadow Priests - unlike their brothers,who decided to walk in an opposite direction,these individuals decided to peer into the void and embrace it's powers to vanquish their foes.They walk on an edge of sanity,and use their abilities to break their opponent minds and inflict a pain upon them.
So,here's my own little description of each above mentioned class (sorry,if it sound a bit cheesy). Alas,share your thoughts,which class lorewise can be considered as the most evil.