Dio's whole appeal lies in how thoroughly he commits to being a complete bastard from day one. Most villains have some tragic backstory or a warped ideology that explains them. Not Dio. He's just pure, unadulterated ambition and spite wrapped in a fabulous blonde package. That's what makes him an antihero to some, I guess—he doesn't pretend to be anything else. He's charismatic, sure, and his flamboyant confidence is entertaining, but it's never for a 'good' cause. He's the living embodiment of 'the world is my property' and he'll step on anyone, betray any ally, to get what he wants. There's no redemption arc waiting.
His longevity across generations in 'JoJo's Bizarre Adventure' cements it. He's not just a one-off bad guy; he's a corrupting force, a meme, a standard. People root for his style, his quotes, his sheer audacity, not because he's secretly noble. He's the villain you love to hate, or maybe just love because he owns it so completely. The 'antihero' label feels like a stretch unless you're using it for anyone who's compelling despite being awful. Dio's defining trait is that he's awful and he's proud of it. It's refreshing in its own horrifying way.