Why the villain?
Villains are free.
Villains are unbound by social constraint, that’s how you know they’re villains.
That’s the fun of it all.
Heroes are boring.
Until the hero crosses the threshold to the magical world — the world-in-itself — she’s bound by convention, expectation, tradition.
In the magical world, she experiences true freedom for the first time. The Campbellian road of trials is a series of opportunities, not to Do Hard Work or to Learn Valuable Lessons, but to experience meaningful freedom, which is responsibility, connectedness, a fullness of engagement. Acting freely in a world unveiled.
(Inaction isn’t freedom.)
Until the hero comes into her power, she is reactive. She takes orders — not least from the villain.
The villain acts, the hero reacts. Until hero and villain square up, the villain represents a problem whose parameters aren’t yet known. The hero can’t imagine how to beat the villain because she can’t imagine being so free.
Out in the magical world, she begins to fully inhabit her actions — she comes, not into power, but into will.
Now her mind is free, her actions are entirely hers. She sees that she is responsible only to herself, and freely chooses to be part of life for reasons other than accumulation and aggrandizement (i.e. she chooses ‘good’).
She returns to the mundane world and acts according to her own will rather than the villain’s. In service.
The villain is revealed in this, the final act, as pitiable or pathetic — because we see hero and villain granted the same power, the same opportunity, but only the hero choosing something larger than herself. The big ‘Let me tell you my evil plan’ speech functions precisely to deflate the villain as character (mad, needy, craven, vengeful, hubristic, etc.) just at his moment of maximum threat to the hero/world/order. In the final act, as the villain grows desperate to stop the hero’s resurgence, he shrinks as she grows. He ends up a solved problem.
The villain is the hero as she might have been, which is to say, hero and villain together constitute a personality. Without a good villain, a hero has (is) no personality at all; she’s inevitable, entitled. A legacy admit.
(One modern convention depicts the villain as a literal element of the hero’s psychology — a personality at war with itself. ‘Clever,’ but one difficulty with such stories is: why is anyone else even in the story?)