Sauron & Saruman spring to mind. We didn’t see or hear much about them before they turned to eeevil, but they definitely started out as good guys.
Khan Noonian Singh only wanted order! And concubines. But mostly order.
Can I call a “reverse Dark Phoenix” on Emma Frost, formerly the Hellfire Club’s White Queen?
Also, I forgot to mention Mystique.
I love it when good guys go temporarily bad. It’s a theme. Good guys always hold back their abilities for fear of hurting someone, but as a fan-boy, I’m always thinking, “but if you’d just used your powers like I know you can, you’d have kicked ass!” So the writers make the good guy temporarily bad and they do use their powers like we know they could. Fun stuff.