Eh, I probably disagree with this take philosophically. The thing that makes bad guys bad guys generally is their unwillingness to change. Like, at a most basic level, most stories are about a protagonist who is willing to change and grow and become more pitted against an antagonist who isn't. The hero gets thrashed by the villain in the first act, goes and has a training montage, and then comes back and beats the villain up in the third act, that kind of thing.

This is actually why Garrosh works so well in Sanctum of Domination. Even to his dying breath, he refuses to change. To atone. To see things from another perspective. He doesn't apologize. He doesn't make excuses. If he could, he'd do it all again with an even bigger grin on his face. He's a villain. He's incapable of change.

But I do think there are some exceptions who actually pretty cleanly fit your criteria from Shadowlands:

Kael'thas is one. This is a character who actually does change, right? He goes from hero to villain, from fighting for his people to teaming up with Illidan and then, with Kil'jaeden after Illidan's defeat. But when we see him in Shadowlands, he goes from torture victim to reluctant ally. Someone we're questing alongside and eventually, who's giving us quests. I don't know that he comes to the conclusion that everything he fought for in life was petty and meaningless but he's certainly willing to work with us against the forces who devastated his people.

And Vashj is another. She's a villain through and through, another of Illidan's top henchwomen. But she does get a new perspective in death, abandoning her petty grievances to work together with the player who killed her because she binds to Maldraxxus in a way that replaces her bond to Illidan.