I do have a couple projects with the villain as protagonist inspired after watching The perfumist.
Many times the villain has a better backstory, or more tragic, or seen as his past crush him into evil makes him feel more human and more relatable.
Or the simple fact that a villain have more freedom because he don't go by the rules (that's why antiheroes are also more likable than normal heroes)
Strenght, you see a villain taking on several main heroes at a time, but a hero can only take on several minions, you never see a hero taking on several final villains.
And more importantly, design. Heroes are normaly a boring looking human, while villains are dragons, robots, intimidating creatures or a cool looking dude with a cape.