Since no one has mentioned it already, I guess I'll throw the first stone: Illidan
Yes, Illidan always cared for power, he always wanted to be stronger and he wasn't above working demons, what's supposed to be his arch-nemesis (both as a demon hunter and as a Night Elf), to get it. However, he did truly want to help his people, he would let Sargeras remove his eyes and replace them with orbs of fire to get the power to save them. He's definitely in the gray area when it comes to morality but he ultimately wishes the best for his own people. That is, naturally, until they choose that he is either supposed to be locked and caged in a dark hole or dead. The Frozen Throne intro really says it all, he is called betrayer but he was in fact betrayed by his own people. And from there, he just slowly descended into madness until he was put out of his misery.
And since we're on the topic, I might as well just add Kael'thas. He was good, he did everything in his power to help a superior who absolutely hated him and his kin, and when that proved impossible and he had to accept other help to make sure he could establish safety for the very people who hated him, he was shunned and caged. He was a hero, but he was not treated as such. Then Illidan showed him a way, he gave him the ability to secure a future for his people, he was their savior. Again, that was ended when he received his almost-mortal wound by Arthas and turned to insanity, and Kael'thas took the exact same path as his supposed master and started making deals with demons in order to get more power.
TL;DR Both Illidan and Kael'thas were good guys who were turned to evil by the people they were trying to protect.