He is the book's main villain, but the Wizard is also a kind of sympathetic character, at least in Elphaba's vision-quest, universe-hopping, drug-induced dreams. In these visions, the Wizard is a kind of pathetic, unemployed fellow, discriminated against for being Irish and driven to attempt suicide by drowning. But all that changes when he gets to Oz, of course.
The Wizard is a classic psychological profile of a nutcase dictator: he's utterly selfish, incapable of thinking of others. His actions speak for themselves: his casual murder of Sarima's entire family, his cruelty toward Nor, his oppression of the Animals, his role in Fiyero's murder. But it's the Wizard's words on his role as a "leader" and his actions that give us chills:
"What I did, what I do, cannot be murder. For, coming from another world, I cannot be held accountable to the silly conventions of a naive civilization. I am beyond that lisping childish recital of wrongs and rights." His eyes did not burn as he spoke; they were sunk behind veils of cold blue detachment. (5.4.55)
One of the main definitions of evil in this book is a refusal to accept personal responsibility. The Wizard refuses to admit he's done anything wrong and has a justification for everything. It may be this refusal to take ownership for his actions, even more than the terrible actions themselves, that makes the Wizard a truly evil character.
The only account we get of a possible break in his character's evil routine is potentially fake:
It may merely be apocryphal that when the Wizard saw the [Miracle Elixir] bottle he gasped, and clutched his heart. (5.18.11)
The Wizard does abdicate the throne, but it's a really hollow victory. He's never brought to justice for all the havoc he wreaked on Oz.