Developing the villain as much as the hero, and showing what ultimately makes the hero what he is, as opposed to just telling you, - this one is just inherently good - that one just inherently evil, is actually a great tool. He blurs the lines of both, and instead shows you anyone could go either way, so what defines them is not what they were, but their choices on the way, that's what makes them different.
That he goes as far as showing how Billy at first empathises with the "villain" actually connects more with him, makes them both more interesting, less black and white, so you actually care about them both!
Ultimately the arc was about Billy being able to share power, and allowing himself to be part of what he feared losing the most, a real family. Something Teth Adam, could sadly never do again. But you need both sides, otherwise what Billy does, what he becomes in the end is meaningless.