Ma'ar/Mornelithe Falconsbane started as being a Well-Intentioned Extremist, but quickly became the Big Bad, and the final showdown between him and the Big Good Urtho caused the Cataclysm, which was so powerful that it actually echoed through time. Though defeated, he didn't actually die, as he had set up a Grand Theft Me so that he could take over the body of any his descendants once they started showing signs of magical ability, most often when they tried to call up fire. Falconsbane kept doing this over the centuries until finally the main characters in the Mage Winds trilogy figured out what was going on and destroyed him.
Ancar kills his father in order to take the throne of Hardorn, tortures Queen's Own Talia (who was there as part of a diplomatic mission to see if a marriage could be set up between Ancar and Princess Elspeth) more for kicks than to get information out of her, and then has his mages mind-control the conscripted troops so that they'll fight. He then drains his land to further power his spells, and kills off even his distant cousins to prevent anyone from overthrowing him.
Hadanelith, a minor villain who appears in The White Gryphon, possesses the Gifts of Empathy and Mindhealing, but uses them to serve his own sadistic desires rather than to aid people. When first discovered, he is found to have been using his gifts to warp the minds of women until they live only to serve him in slavery, sexually and otherwise. He is exiled from the city of White Gryphon under the relatively loose laws and customs of their society. Rather than die in the wilderness, he makes his way south to the Haighlei kingdoms and is recruited by the Evil Chancellor to assassinate high ranking members of their society in order to frame the delegation from White Gryphon. In doing so, he is permitted to indulge his sadistic fantasies and accordingly tortures his victims before killing them.