What is the most evil thing you ever did as a DM? The most cruel thing I can remember was having a player encounter one of his most beloved characters, one who had been captured by mind flayers, and encountering his own character, years later, both irl and in game, who had been enslaved for years, and reduced to the level of a retard, but very, very slowly, and begged his own player to kill him.
The character had started out as a genius and the mind flayers loved tormenting him by turning him into a retard.
This player decided his goal was to eliminate mind flayers from existence and he achieved this. By the time he was done, not only did illithids no longer exist (pure genocide achieved), but psionic abilities were entirelely outlawed and there was basically a Butlerian jihad.