Power

Power not only corrupts, it also attracts others who seek to have it.

There are other mages out there.

People will want to overthrow an open tyrant. Other mages can use this to their advantage and inspire such a rebellion to take that mage's place.

Mages not just powerful, they are smart too.

In Warcraft III the Frozen Throne (just before the first World of Warcraft) Illidan recruited Broken Draenei into Illidari promising to rid the Outland of Magtheridon and his tyranny. And did at least the first part, becoming new Lord of Outland in the process.

Only later in WoW Illidan was made into a villain. Before he was grey misunderstood character who did many good things like killing Tichondrius to help stop the Legion before the battle for Mount Hyjal or helping Kael'Thas Blood Elves to escape an unjust execution by the racist Alliance commander Garithos. He even saved Tyrande from Scourge while so called 'good' Maev just left her to die. But most of it went unappreciated. Draenei could have also end up like Fel Blood Elves and Naga: one of the staples of new order in Outland. Only in WoW they ended up enslaved.

Hershel in Clash of Mages, much like Illidan in Warcraft also a gray character who uses cunning and wit in addition to his magic skills to obtain what he wants, but does good things too. Just like Illidan or Lelouch in Code Geass it was his tragic past that made him this way.

Illidan assaults Black Citadel that later in WoW became Black Temple
https://www.youtube.com/watch?v=RreF6...

And the Clash of Mages:
https://www.amazon.com/Exploits-Party...