Fastrin

Fastrin the Delver was a powerful and good wizard and ruler of an ancient kingdom in the Sunrise Mountains in Thay. That kingdom came to existence not long after the fall of the Netheril. One day though, Fastrin went mad and started to slaughter his people. When asked for the reason of his menace, Fastrin claimed he had been robbed and now tried to find and kill the thief. Since he couldn't find the thief, Fastrin believed the death of everyone of his realm to be the only way to be safe. He not only killed many of his people but mangled the minds and spirits of others, stripping them of memory and reason. Some of them tried to stand against Fastrin and reappered as ghosts after their deaths, wandering the land without memory and purpose, like the ghost Mirror. After destroying his kingdom, Fastrin obliterated himself.

The reason why he had to kill everyone in his realm according to his opinion, was that the stolen item was the book that contained the ritual "the unmaking" which could destroy the actual universe and recreate it. (or let it be completely obliterated)

So in a sense, he obliterated everyone for the greater good. He didn't really become mad, but decided that it was too much a threat to leave the book in unknown hands.